fix(platform): document isEmbed and guard platform embed header

[hackice20]

What does this PR do?

Documents the isEmbed prop in CalProvider, clarifies when to use it, and hardens the API so x-cal-platform-embed: true only bypasses OAuth client metadata when the request truly comes from Cal’s managed embed OAuth client.

• Fixes refactor: CalProvider isEmbed #24966

Visual Demo (For contributors especially)

Image Demo (if applicable):

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

• Docs
  Run mintlify dev under docs/.
  Open /platform/atoms/cal-provider and verify isEmbed appears in the props table with the warning.
• API
  wrote a test file to check this , my laptop can't handle the load to run the backend.

Checklist

• I have read the contributing guide
• My code does follow the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have checked if my changes generate no new warnings

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[cubic-dev-ai]

No issues found across 6 files

[hackice20]

Hello @anikdhabal can you please review this PR ? :)