CL-8905: lock MCP promotion schema fidelity for optional and required arguments

[TheGreatAxios]

Test-only regression guard for CL-8905 (corbits-code MCP side).

Scope: corbits-code MCP plumbing only. Canceled children CL-8906/CL-8907/CL-8908 untouched; tools-linear untouched; isError/structuredContent surfacing left to in-flight CL-8992.

What this lands:

• Prior commit d97cf3a proved optional MCP properties survive tool_search promotion with no 'required' key synthesized on the wire.
• Commit a38e29c added a second promotion test with a mixed schema (required: [limit] plus optional team/customView) asserting the published wire input_schema is deep-equal to the server schema and dispatched args arrive exactly as the caller specified.
• New commit ae133d3 hardens the guards per review: two loud-failure tests (unadvertised tool dispatch and out-of-scope server rejection, both asserting isError), optional args transmitted on both promotion dispatches ({limit: 1, team: "eng"} and {limit: 1, team: "eng", customView: "mine"} with full receipt asserted), and schema expectations snapshot-cloned before addTools so in-place mutation cannot move both sides.

Per-box evidence (corbits-code-owned paths):

• Box 1 (direct queries express only caller-requested filters): both promotion tests assert receivedArgs deep-equals the caller's full args; wire schema is byte-faithful, so the model sees exactly the server's filters.
• Box 2 (invalid/unintended scope cannot silently alter results): 'unadvertised MCP tool dispatch fails loudly' asserts unknown-tool content with isError true on the live-dispatch path (frozen-path test asserts the same isError); 'out-of-scope MCP arguments fail loudly' asserts a server scope-denied rejection surfaces as isError with the actionable message. Wire schema can neither gain nor lose required-ness silently — locked by the deep-equal/snapshot assertions.
• Box 3 (effective scope visible): promoted tool schemas are published in full on the next infer, asserted via captured Anthropic request bodies.

Red evidence (targeted mutation check in src/mcp/plugin.ts mcpClientTools passthrough, restored after):

• Injecting required: [] on the all-optional schema turned the optional-promotion test red (hasOwn required true, expected false); restoring returned it green.
• Deleting required on the mixed schema turned the mixed-promotion test red (snapshot deep-equal diff on required: [limit]); the all-optional test stayed green under the same mutation, proving targeting; restoring returned all green.
  No src changes needed — plumbing passes definitions/args by reference end to end.

Verification:

• bun run typecheck: exit 0
• bun test tests/integration/mcp-late-dispatch.test.ts: 6 pass, 0 fail
• bun run lint: exit 0
• bun test ./src ./tests ./evals ./scripts --randomize --seed 424242: 8031 pass, 0 fail

[linear-code]

CL-8905