test: assert RuntimeContext fields are populated per task (Closes #149)

[rafaio1]

Summary

Adds comprehensive test coverage for RuntimeContext field population during task execution, addressing bounty issue #149 ($85).

Changes

• New test file: tests/runtime/context-fields.test.ts
• Validates that all required context fields (taskId, sessionId, timestamp, agentId) are correctly populated
• Covers both runtime startup and active task execution scenarios
• Tests field immutability and type safety

Testing

All tests pass locally. No CI configured on upstream repo.

Bounty

Closes #149 ($85) — uncontested bounty for RuntimeContext field validation testing.

/cc @Lilly-Protocol