MCP invocation falls through empty OAuth2 token to HTTP field, diverging from discovery #7646

Description

Follow-up to #7587 / #7590.

In McpTool._get_headers(), the OAuth2 branch currently reads:

if credential.oauth2 and credential.oauth2.access_token:
    headers = {"Authorization": f"Bearer {credential.oauth2.access_token}"}
elif credential.http:
    ...

When an AuthCredential carries an OAuth2 object with an empty/None access_token and an HTTP credential on the same object, the OAuth2 condition is falsy, so control falls through to the elif credential.http: branch and emits an Authorization header from the HTTP field.

The canonical discovery helper build_auth_headers() (used by McpToolset) instead keeps the credential on the OAuth2 branch and emits no header for an empty token:

if credential.oauth2:
    if credential.oauth2.access_token:
        headers = {"Authorization": f"Bearer {credential.oauth2.access_token}"}
elif credential.http:
    ...

Impact

Discovery (McpToolset._get_auth_headers → build_auth_headers) and invocation (McpTool._get_headers) resolve the same credential to different Authorization headers for this mixed shape. Both feed _merge_auth_and_provider_headers, and the MCP session pool key is derived from the merged headers, so the two paths can bind to different pooled sessions — the path-consistency concern that motivated #7548 / #7550.

This is reachable: a custom auth provider may return such a mixed credential (the non-consent path in CredentialManager returns it without clearing fields). It is a narrow edge, not the default OAuth2/HTTP flow.

Reproduction

With an OAuth2 credential whose access_token is empty and an HTTP bearer field present, invocation yields Authorization: Bearer <http-token> while discovery yields no OAuth header. A focused regression test at the create_session() boundary fails on current main with:

Expected: create_session(headers={'Authorization': 'Bearer provider-token'})
Actual:   create_session(headers={'Authorization': 'Bearer stale-http-token'})

Proposed fix

Restore the nested structure so the OAuth2 credential stays on its own branch and an empty token simply emits no header, matching build_auth_headers(). One-line structural change plus a regression test for the mixed-credential case.

I reproduced this on current main and have the fix and verification ready; I'd like to take this follow-up.