[Bugs] OAuth flow never triggers for BigQuery MCP — oauthClientId config has no effect

## Description

When configuring a remote MCP server with `oauthClientId` in `mcp-config.json`, the expected behavior is that Copilot opens a browser window to authenticate the user via OAuth. This works correctly for some servers (e.g. Datadog), but **never happens for BigQuery** (`https://bigquery.googleapis.com/mcp`).

From the user's perspective:
- The config looks correct
- No browser window ever opens
- Every tool call silently fails with an authentication error
- There is no feedback explaining why OAuth didn't trigger or what to do

The only workaround discovered is to manually inject a static `Authorization: Bearer ` header — which expires every hour and requires manual renewal. This is not a viable long-term solution.

## Steps to reproduce

1. Add BigQuery MCP to `mcp-config.json` with `oauthClientId`
2. Restart Copilot
3. Try any BigQuery tool → auth error, no OAuth prompt

**Expected:** Browser opens for Google OAuth consent, token stored, tools work
**Actual:** Silent failure, `oauthClientId` has no effect

## Related

#306

Technical context (unverified)

The OAuth flow appears to only be triggered when the initial `tools/list` request returns a 401. BigQuery MCP allows `tools/list` without authentication, so the connection is established without an auth provider. When `tools/call` later returns 401, there is no mechanism to recover via OAuth at that point.