fix(web_core): accept function calls that omit args (v0.9)

[wrenj]

Problem

common_types.json#/$defs/FunctionCall requires only call, but the Zod
schema marked args as required. So a spec-valid {"call": "now"}:

• failed validation, and
• crashed DataContext with a raw TypeError from
  Object.entries(undefined) on both the sync and reactive resolve paths.
  Swift already models args as optional, so web_core was the outlier.

Fix

• FunctionCallSchema: args is now .optional(), matching the JSON schema.
• DataContext: default to {} in resolveDynamicValue and resolveSignal.
  The optional type makes removing either guard a compile error.

Tests

Four added, each confirmed to fail before the fix:

• data-context.test.ts: argument-less calls resolve on the sync and
  reactive paths, and the invoker receives {}.
• verify-schema.test.ts: FunctionCall.required matches the spec JSON, and
  an argument-less call parses. The existing parity suite only compares
  server_to_client definitions, which is why this drift went unnoticed.

[gemini-code-assist]

Code Review

This pull request makes the args field optional in FunctionCallSchema to align with the specification. It updates DataContext to handle omitted arguments using nullish coalescing (call.args ?? {}) when resolving dynamic values and signals, and adds corresponding unit tests to verify this behavior. There are no review comments, so I have no feedback to provide.