Handle absent tool observability during upload

[chubes4]

Summary

Keep optional tool observability absent during reviewer transcript staging.

What changed

• Guarded the optional projection before array/object emptiness filtering and added native Playground coverage for runs without tool observability.

How to test

1. Run npm run test:native-agent-task-playground-e2e; expect The formerly failing upload path succeeds and omits absent tool observability..
2. Run npm run build; expect TypeScript compilation succeeds..
3. Run npm run test:agent-task-contracts; expect Agent-task and runtime materialization contracts pass..
4. Run npm run test:browser-runner-template; expect Browser runner template contracts pass..

Compatibility

Bug fix restoring the pre-#1809 behavior for runs that do not expose optional tool observability. Runs with canonical version 1 observability are unchanged.

Evidence

• CI expected: Agent Task Contracts / contracts
• Reviewer-resolvable source evidence: Reviewer transcript projection crashes when tool observability is absent #1810
• agent-task-contracts: Passed
• browser-runner-template: Passed
• build: Passed
• native-playground: Passed

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode
• Model: openai/gpt-5.6-sol
• Used for: Diagnosed the post-merge CI failure, implemented the null-safe optional projection, added native Playground regression coverage, and verified all affected contracts; Chris reviewed and owns the change.

Source relationships

• Closes Reviewer transcript projection crashes when tool observability is absent #1810