require_admin_token uses inspect_err to log non-UTF-8 header encoding issues,
but require_pending_session silently dropped such errors with no logging.

Applied the same pattern from require_admin_token to require_pending_session
to ensure consistent behavior across both auth functions.

- Critical #1: Retry path ignores pre-stored pending_did
Added comparison between derived DID and pre-stored DID on retry path.
If they don't match, return InternalError explaining the mismatch.
This prevents undetected DID mismatches when client inputs change between attempts.

- Critical #2: PLC directory response body never logged
After checking !response.status().is_success(), now consume the response body
with response.text().await and include it in the tracing::error! log.
Operators will now see the actual error response instead of just the HTTP status.

- Updated retry test to pre-store the actually-derived DID so it matches
what the handler will re-derive on the retry path.

- [Critical] Fix formatting in auth.rs (lines 91-128) and create_did.rs
(multiple locations): run cargo fmt --all to auto-format both files
to comply with CI gate requirements.

- [Minor] Remove unnecessary #[allow(dead_code)] from AppState.http_client
field in app.rs line 81. The field is actively used by create_did_handler
at create_did.rs:161 for plc.directory HTTP calls.