refactor(tests): consolidate Dispatch Pipeline Module — 7 files → 1 (#3733)

Consolidates 7 sibling test files for sdk/src/query/query-dispatch.ts and
its stage handlers into a single query-dispatch.test.ts with 8 describe
blocks (699 LOC, 53 tests). Deletes 3 clean shim sources with zero non-test
importers. Leaves query-dispatch-formatting.ts and query-dispatch-error-mapper.ts
in place (non-test importer: query-fallback-executor.ts — deferred to #3732).

- query-dispatch-input-validation.ts deleted (clean shim)
- query-dispatch-plan.ts deleted (clean shim)
- query-dispatch-result-builder.ts deleted (clean shim)
- 6 per-stage test files deleted (consolidated into query-dispatch.test.ts)
- counter-tests added per Contract 6 for each stage field
- CONTEXT.md Glossary updated with Dispatch Pipeline Module entry

Closes #3731

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>