google-dialog: Report Vertex AI auth failures and route Agent Platform correctly

[pragmatrix]

Why

Agent Platform (Vertex AI) sessions reported credential problems as a generic
SetupFailed error, so a missing or invalid Application Default Credentials
(ADC) setup looked like a setup or handshake failure with no indication of how
to fix it. The Agent Platform endpoint was also hard-coded to
{location}-aiplatform.googleapis.com, which is not the host that serves the
us/eu multi-regions or global.

Summary

• google-dialog resolves Vertex AI ADC before connecting (bounded by a 10 s
  timeout matching the transport's connect timeout), so a credential problem is
  reported as an auth failure instead of a WebSocket handshake or setup failure.
• Auth failures (bearer-token errors and 401/403 handshake rejections) are
  annotated with an ADC hint naming the fix (gcloud auth application-default login, or GOOGLE_APPLICATION_CREDENTIALS). The hint applies to the initial
  connect only.
• Agent Platform routing uses the transport's Endpoint::VertexAi, which selects
  the correct host: us/eu → aiplatform.{location}.rep.googleapis.com,
  global → aiplatform.googleapis.com, other regions →
  {location}-aiplatform.googleapis.com.
• Submodule gemini-live-rs bumped to de2e3e0: connect errors are forwarded to
  the client via SessionError::Connect, handshake rejections carry the endpoint
  URL with its query stripped (so credentials are not leaked), and the Vertex host
  routing above is implemented.
• Workspace version bumped to 3.10.0 with a VERSIONING.md release entry.

Validation

• cargo fmt --all -- --check — clean
• cargo clippy -p google-dialog --all-targets -- -D warnings — clean
• cargo test --workspace — green

Note

This was created entirely with AI assistance.