[API] Add type index query mode

[tqchen]

Summary

• Add query-only behavior to TVMFFITypeGetOrAllocIndex when parent_type_index == -2.
• Return the existing registered type index on hit, or -2 on miss without allocating/registering or mutating parent metadata.
• Document the concrete -2 C API behavior and add focused C++ coverage for hit and miss/no-registration behavior.

This query mode is reserved for a future release update that may optionally query type indices at startup.

[gemini-code-assist]

Code Review

This pull request introduces a query-only mode to TVMFFITypeGetOrAllocIndex by using a parent_type_index of -2. When this value is passed, the function queries the existing type index for the given type key without allocating a new one, returning -2 if the key is not registered. The changes include updates to the C API documentation, the implementation of QueryTypeIndex in TypeTable, and corresponding unit tests to verify the new behavior. No review comments were provided, so there is no feedback to address.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.