[jterapin]

Description

Failures in response callbacks or body cleanup can be lost after completion. Late headers or data can also modify a completed response. HTTP-method lookup can resolve constants that are not request classes.

Customer impact

A callback failure is no longer silently treated as success. Retry eligibility follows V3's error classes and HTTP status rules.

Fix

• Preserve the first response failure, run completion cleanup once per attempt, and notify error listeners after completion callbacks.
• Use the V3 network-error classes when handling caught transport, callback, and body-processing exceptions. Preserve other exceptions and retain the original cause when wrapping a network error.
• Ignore headers and data after completion, and reset response state before another attempt.
• Reject HTTP-method constants that are not Net::HTTP request subclasses.
• Simplify transport dispatch, clarify the touched comments, and correct the networking-error and backoff signatures.

Testing

Local checks ran on Ruby 4.0.4. AWS V4 validation used the modified Smithy runtime through Bundler local overrides.

• smithy-client specs: 951 examples, 0 failures, 2 existing pending.
• RBS validation and spy specs: 901 examples, 0 failures, 2 existing pending.
• AWS V4 protocol suites: 1,106 examples, 0 failures, 4 existing pending.
• RuboCop: 374 files inspected, no offenses. git diff --check passed.
• An offline comparison against V3 matched all 15 callback/error/status cases. Mocked S3 calls verified callback errors, retries, cleanup counts, and retry limits.

🤖 This PR description was drafted with AI assistance and reviewed by @jterapin. ✨