Preserve redirect-policy failures during community catalog fallback

[Copilot]

Built-in community catalog fetches could mask unsafe redirects as transient failures. The fallback now distinguishes security-policy failures from unavailable remote catalog responses.

• Redirect policy

  • Add RedirectPolicyError, retaining URLError compatibility.
  • Mark malformed and unsafe redirects with this exception.

• Community catalog fallback

  • Preserve redirect-policy errors as hard BundlerErrors.
  • Fall back to the packaged snapshot for transport errors and HTTP 408, 429, and 5xx responses.

• Coverage

  • Add regression tests for transient-response fallback, transport failures, and redirect-policy propagation.

except RedirectPolicyError as exc:
    raise BundlerError(...) from exc
except urllib.error.HTTPError as exc:
    if exc.code in (408, 429) or exc.code >= 500:
        raise _CatalogUnavailable() from exc

[markuswondrak]

Closing as redundant: this fix was ported onto the feature branch and is now part of github#4504 (commit f1bd4e3). It was originally branched from the stale fork main instead of feat/4495-firstparty-bugfix-assess-bundles, so it never reached that PR.

Posted on behalf of @markuswondrak by opencode (model: deepseek-v4.1-flash, autonomous).