fix(api_core): clarify misleading http 404 unimplemented error message

[guptaarima]

Summary of the issue:

Context:
When a gRPC call fails with grpc.StatusCode.UNIMPLEMENTED but the error details string includes "Received http2 header with status: 404", the google-api-core library currently translates this into google.api_core.exceptions.MethodNotImplemented. This scenario often happens due to client-side configuration errors like an incorrect api_endpoint or a malformed resource path, not because the method is actually unimplemented.

Actual Behavior:
The library raises google.api_core.exceptions.MethodNotImplemented: 501 Received http2 header with status: 404. This exception is misleading because the primary issue indicated by the underlying error is related to the 404 (Not Found), not that the method is unimplemented (501). This makes debugging difficult for users.

Expected Behavior:
google-api-core should raise an exception that better reflects the underlying HTTP 404. Suggested improvements:

Prioritize the 404 status when present in the details, raising google.api_core.exceptions.NotFound (404).

The goal is to provide a more actionable error message to the user, guiding them to check the endpoint or resource path.

Fixes b/362438722

[gemini-code-assist]

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[parthea]

Thanks! I edited the PR description to remove the alternative suggestion of raising ServiceUnavailable (503). Your actual code implementation of keeping the exception type but enriching the error message is much better.

⚠️ One Small Caveat to Keep in Mind

This is technically a weak breaking change for exception messages. While substring assertions (e.g., assert "404" in str(e)) will continue to pass because we prepend the original message, any brittle test suites in downstream projects doing exact string equality matches on this exception message will fail.

Given how much this improves developer troubleshooting, I think this soft risk is entirely acceptable, but it is worth noting.

I'll add formal approval once tests pass.