fix(stdio): treat "id": null as a request, not a notification

[dpage]

Summary

• Filter JSON-RPC notifications at the stdio read loop using
  hasIDField on the raw line bytes, matching the HTTP transport fix
  from HTTP transport: server replies to JSON-RPC notifications with malformed (no-id) Response objects #142/fix(http): return 202 Accepted with empty body for JSON-RPC notifications #143. Notifications now never reach dispatch.
• Remove the now-redundant req.ID == nil guards in
  handleRequest's default branch and handlePing; a request with
  explicit "id": null now reaches the dispatcher and is answered.
• Drop the hardcoded notifications/initialized case from the
  dispatch switch (filtered upstream, and a request with that method
  and id: null was being silently swallowed).

Test plan

• go test ./internal/mcp/ — added Run()-level tests that pipe
  stdin and assert the resulting stdout
• notifications/initialized produces no response
• Unknown notification methods (/cancelled,
  /roots/list_changed, some/unknown) produce no response
• {"jsonrpc":"2.0","id":null,"method":"unknown/method"} receives
  -32601 Method not found
• {"jsonrpc":"2.0","id":null,"method":"ping"} receives an
  empty-object result
• make build, make test, make fmt all clean. make lint
  reports 3 pre-existing staticcheck findings in
  internal/embedding/{ollama,provider}.go (untouched by this PR;
  present on origin/main).

Closes #152

Summary by CodeRabbit

• Bug Fixes

  • Improved JSON-RPC stdio behavior: requests with explicit null IDs now return proper error responses for unknown methods instead of being dropped; initialization and other notifications are consistently filtered and produce no responses.

• Documentation

  • Changelog updated to document the notification vs. request differentiation for stdio transport.

• Tests

  • Added end-to-end stdio/RPC tests validating notification filtering and null-ID request handling.

[coderabbitai]

Walkthrough

This PR fixes a JSON-RPC 2.0 compliance bug in the stdio transport where requests with explicit "id": null targeting unknown methods were silently dropped instead of returning a -32601 Method not found error. The fix reuses the raw-bytes hasIDField probe to distinguish notifications (no id field) from requests (field present, including null values), updates request dispatch to unconditionally send errors, and adds comprehensive integration tests.

Changes

JSON-RPC stdio dispatch compliance

Layer / File(s)	Summary
Notification detection and request handler updates internal/mcp/server.go	STDIO loop now reads raw scanned bytes and uses hasIDField to detect presence of an id member. Notification filtering moved into Run. handleRequest and handlePing no longer skip responses when req.ID is nil; unknown-method errors are sent for requests (including "id": null). Comments updated accordingly.
Integration test coverage for stdio dispatch internal/mcp/server_test.go	Adds runStdio helper that pipes JSON-RPC lines into Server.Run() and captures stdout. Adds tests asserting notifications/initialized and other notifications produce no stdout, and that requests with explicit "id": null get -32601 for unknown methods or a proper ping response. Removes an earlier unit test that directly invoked handlePing.
Changelog documentation docs/changelog.md	Unreleased changelog documents the stdio behavior change: absent id = notification suppressed; explicit "id": null = request that must be replied to; notifications/initialized is now filtered uniformly via hasIDField.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• pgEdge/pgedge-postgres-mcp#167: Related changes to stdio JSON-RPC dispatch and handlePing around nil vs null id handling.
• pgEdge/pgedge-postgres-mcp#143: Introduced the hasIDField raw-bytes approach for the HTTP transport to distinguish absent id from null; this PR applies the same approach to stdio.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: fixing the stdio transport to treat requests with explicit id:null as requests rather than notifications.
Linked Issues check	✅ Passed	The PR fully addresses issue #152 requirements: distinguishes notifications from id:null requests via hasIDField, returns -32601 for unknown methods, and adds comprehensive Run()-level tests.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to issue #152: stdio transport bug fix, documentation updates in changelog, and targeted test coverage with no unrelated modifications.
Docstring Coverage	✅ Passed	Docstring coverage is 83.33% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-152-stdio-null-id

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[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 23 complexity

Metric	Results
Complexity	23

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@internal/mcp/server.go`:
- Around line 223-228: The JSON-RPC ping handler (handlePing -> sendResponse)
promises to reply even when req.ID is a nil interface (incoming "id": null), but
JSONRPCResponse.ID in internal/mcp/types.go is tagged `json:"id,omitempty"` so
marshaling will drop the id field; remove `omitempty` from the JSON tag on
JSONRPCResponse.ID (or implement custom MarshalJSON for JSONRPCResponse to
explicitly emit "id": null when ID == nil) so responses to null-id pings include
`"id": null`, and update the stdio tests to assert the raw JSON contains `"id":
null`.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 18ceb9d8-0db0-4776-a611-444d0a9a85e8

📥 Commits

Reviewing files that changed from the base of the PR and between 317aa48 and 2406292.

📒 Files selected for processing (2)

• internal/mcp/server.go
• internal/mcp/server_test.go

🚧 Files skipped from review as they are similar to previous changes (1)

• internal/mcp/server_test.go