Use structured logging for FetchFulfillment errors

[jonathangreen]

Description

Replace the unstructured error log message in FetchFulfillment with structured palace_-prefixed fields, making fulfillment failures much easier to query and aggregate in CloudWatch Logs Insights.

Previously, the upstream response body, status code, and URL were all embedded in a single log message string, requiring parse expressions to extract in CloudWatch. Now they are emitted as separate structured fields:

• fulfillment_url — the upstream URL that failed
• fulfillment_status_code — the HTTP status code from upstream
• fulfillment_response — the response body, parsed as a JSON object when possible, raw string otherwise
• fulfillment_headers — the full response headers as a dict

The human-readable log message still includes the URL and status code for quick scanning.

Motivation and Context

While investigating audiobook fulfillment failures reported via Crashlytics, we found that the existing log format made it difficult to query and aggregate errors in CloudWatch. Extracting the upstream response from the log message required fragile parse expressions, and the response body couldn't be inspected structurally when it was JSON (e.g., OPDS problem detail documents).

How Has This Been Tested?

• Updated existing test for JSON error responses to verify structured fields are present on the log record
• Added new test for non-JSON error responses (e.g., HTML error pages) to verify fallback to raw string
• All tests pass, mypy clean

Checklist

• I have updated the documentation accordingly.
• All new and existing tests passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.26%. Comparing base (5f39ae3) to head (c301e60).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3153      +/-   ##
==========================================
- Coverage   93.26%   93.26%   -0.01%     
==========================================
  Files         493      493              
  Lines       45581    45586       +5     
  Branches     6252     6252              
==========================================
+ Hits        42513    42517       +4     
- Misses       1982     1983       +1     
  Partials     1086     1086              

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[tdilauro]

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