fix(router): resolve plan cache memory leaks on config reload

[dkorittki]

Combines #3006, #3007, and #3009 with the following additional changes:

• Do not wait for pending slow-cache writes on mux shutdown, cache is about to be closed anyways
• Reset minKey and minDur fields when closing the slowplancache

#3008 currently not included here, needs some more work. Will be merged separetely.

Summary by CodeRabbit

• Bug Fixes
  • Improved plan cache shutdown so cached entries are fully released and internal tracking is reset.
  • Prevented cache eviction handling from writing entries into fallback storage during router shutdown.
  • Cleaned up plan metadata handling to return only the data that is actually used.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• If applicable, I have provided instructions for reviewers for manually validating my changes.
• Documentation has been updated on https://github.com/wundergraph/docs-website.
• I have read the Contributors Guide.

Open Source AI Manifesto

This project follows the principles of the Open Source AI Manifesto. Please ensure your contribution aligns with its principles.

[coderabbitai]

Walkthrough

The PR updates planner metadata, changes slow-plan-cache shutdown to drop stored entries and reset bookkeeping, and adds a shutdown gate around plan-cache eviction handling in the router.

Changes

Plan cache lifecycle and planner metadata

Layer / File(s)	Summary
Planner metadata cleanup router/core/operation_planner.go	planWithMetaData removes schemaDocument, and planOperation returns the remaining metadata fields.
Slow plan cache close cleanup router/pkg/slowplancache/slow_plan_cache.go, router/pkg/slowplancache/slow_plan_cache_test.go	Cache.Close() deletes stored entries, resets cache bookkeeping, and the new test checks the cache is empty afterward.
Plan-cache eviction hook gating router/core/graph_server.go	graphMux adds an atomic hook flag, enables it for warmup with in-memory fallback, wraps OnEvict with the flag check, and disables it before shutdown closes the plan cache.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• wundergraph/cosmo#2838: Also changes graphMux.Shutdown behavior in router/core/graph_server.go, which is directly related to the shutdown path updated here.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: fixing router plan cache memory leaks during config reload.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

---

_{Comment @coderabbitai help to get the list of available commands.}

[github-actions]

Router-nonroot image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-9869fb2e19cc772d564771f6dcaa050705d42792-nonroot

[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-9869fb2e19cc772d564771f6dcaa050705d42792

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.42%. Comparing base (f144989) to head (b4f4018).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3023       +/-   ##
===========================================
+ Coverage   46.53%   61.42%   +14.88%     
===========================================
  Files        1120      259      -861     
  Lines      154966    30055   -124911     
  Branches    10060        0    -10060     
===========================================
- Hits        72107    18460    -53647     
+ Misses      81025    10107    -70918     
+ Partials     1834     1488      -346     

Files with missing lines	Coverage Δ
router/core/graph_server.go	85.70% <100.00%> (+0.04%)	⬆️
router/core/operation_planner.go	71.11% <ø> (-0.32%)	⬇️
router/pkg/slowplancache/slow_plan_cache.go	99.05% <100.00%> (+0.05%)	⬆️

... and 863 files with indirect coverage changes

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[dkorittki]

@SkArchon can you take a look at this?

[SkArchon]

Tried it locally as well, lgtm