Cover ResponseAwareException fully

[loevgaard]

Closes the coverage gap on ResponseAwareException — previously 4/8 methods and 89.8% of lines; now 8/8 and 100%.

What was uncovered

• getResponse() — never called by any test
• the lazy parse path: parseBody() reading from the response stream when no $body was passed at construction (every SDK-internal throw passes $body, so only direct construction reaches it)
• getErrorCode()'s defensive int→string cast
• getValidationErrors()'s non-map errors branch

New behavior pins

Two of the six new tests assert behavior this class exists for, which was exercised but never asserted:

• Pre-read body wins over a drained stream — the non-seekable-stream robustness described in the class docblock: with an empty response stream, only the pre-read $body keeps the getters working.
• The default message embeds a sanitized request context — query string and fragment stripped, asserted character-exact, so consumer-supplied secrets can never silently start leaking into exception messages/logs.

Result

• Tests: 103 → 109 (PHPStan max, ECS green)
• Mutation score: MSI 80% → 85%, covered MSI 82% → 85% (gate: 70)
• The four remaining escaped mutants in the class are behaviorally equivalent (idempotent re-decode, trims of never-padded strings, sprintf's implicit cast).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.50%. Comparing base (52dd84a) to head (cd28cb4).

Additional details and impacted files

@@             Coverage Diff              @@
##                1.x       #5      +/-   ##
============================================
+ Coverage     97.00%   98.50%   +1.49%     
  Complexity      146      146              
============================================
  Files            24       24              
  Lines           401      401              
============================================
+ Hits            389      395       +6     
+ Misses           12        6       -6     

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