Add Python stdlib and dependency cross-references (objects.inv inventories)

[Alberto-Codes]

Problem

The API reference docs don't link stdlib types (e.g., typing.Protocol, asyncio.to_thread, bytes) or external dependency types (e.g., google.adk.sessions.BaseSessionService, cryptography.fernet.Fernet) to their upstream documentation. Docvet has this via mkdocstrings inventories — we should match.

Solution

Add inventories to the mkdocstrings handler in mkdocs.yml:

inventories:
  - https://docs.python.org/3/objects.inv

Additionally, investigate objects.inv availability for direct dependencies:

• cryptography — https://cryptography.io/en/latest/objects.inv
• aiosqlite — check if published
• google-adk — check if published

Acceptance Criteria

• Stdlib types in API reference link to docs.python.org
• External dependency types link to their upstream docs where objects.inv is available
• mkdocs build --strict still passes