Remove dead retry_on_httpx_errors decorator on metadata fetch

[jp-spotter]

Removes a dead retry decorator on workflows.metadata.fetch.

retry_on_httpx_errors wrapped fetch, but fetch raises ExceptionGroup (via
BoundedTaskGroup), which its retry_if_exception_type((ReadError, ConnectTimeout, HTTPStatusError)) predicate can never match — so it never fired (reraise=True propagated the
same exception either way). It was also redundant with the transport-layer retry in client.py,
which retries every request (idempotency-aware, exponential backoff + jitter, 429/503/502/504).

Pure deletion: the decorator, its definition, and the now-unused tenacity import. No behavior
change — per-request retries remain intact at the transport layer.

Retry / transport / workflow test suites green.