python-v0.144.4: Support custom transports for Amazon Bedrock (#33695)

What changed

• Allow the built-in amazon-bedrock provider to override base_url, auth,
  and http_headers in addition to its AWS profile and region.
• Use command-based bearer authentication and configured endpoints without
  applying AWS request signing, while retaining regional endpoint resolution
  for the default Bedrock configuration.
• Replace the Bedrock account credentialSource enum with the
  usesCodexManagedCredentials boolean so command-authenticated and other
  externally managed configurations are reported consistently.

Testing

• Cover configuration merging and validation, command-authenticated proxy
  requests with custom headers, and account reporting for managed and external
  credentials.

GitOrigin-RevId: d1acbe602060470583b5e12f8d304bee5be46f4c