Surface hasMappedDependants on ts-sdk TaskContext

[bujjibabukatta]

Adds TaskContext.hasMappedDependants to the TypeScript SDK, mirroring
the should_retry field pattern already used for TIRunContext →
TaskContext mapping.

When true, returning an array from a task handler causes the
supervisor to record mapped_length on the return-value XCom
automatically, letting the scheduler expand downstream mapped
dependants — matching behavior already available to Python task
handlers.

Changes:

• ts-sdk/src/sdk/task.ts — add readonly hasMappedDependants: boolean
  to the public TaskContext interface
• ts-sdk/src/coordinator/runtime.ts — wire ti_context.has_mapped_dependants
  into buildContext(), defaulting to false when the server omits it
• ts-sdk/tests/coordinator/integration.test.ts — cover both the
  true and default-false cases end-to-end through the coordinator
• ts-sdk/tests/coordinator/client.test.ts, ts-sdk/tests/public-api.test.ts —
  update existing TaskContext fixtures/type assertions for the new
  required field

Depends on has_mapped_dependants existing on TIRunContext
(#70571) — rebased onto that once merged, schema.json /
supervisor.ts regeneration is a no-op beyond the one field.

closes: #71063

Was generative AI tooling used to co-author this PR?

• Yes (Claude)
• Used for PR description and test cases