Reviewer transcript projection crashes when tool observability is absent

[chubes4]

Regression

After #1809, prepare-agent-task-upload.mjs includes optional tool_observability in projectParsed(), then its Object.fromEntries(...).filter(...) calls Object.keys(item) without handling undefined. Native agent runs without canonical tool observability crash during upload staging.

CI evidence:

• https://github.com/Automattic/wp-codebox/actions/runs/29433410060/job/87413877943
• https://github.com/Automattic/wp-codebox/actions/runs/29433350001/job/87413677263

Observed error:

TypeError: Cannot convert undefined or null to object
    at projectParsed (.../prepare-agent-task-upload.mjs:221)

Required outcome

• Omit absent optional projection fields without calling Object.keys() on nullish values.
• Preserve array/object filtering semantics for present fields.
• Add regression coverage through npm run test:native-agent-task-playground-e2e.
• Re-run build and the agent-task/browser contract suites.

Regressed by #1809.