Support non-rewindable Rack inputs for Events signature verification

[dblock]

Summary

Successor to #515, crediting @olleolleolle for the original conditional-rewind fix. Rack 3 no longer requires rack.input to implement rewind, and rackup/WEBrick exposes a non-rewindable stream.

• Rewind before and after reading only when supported; preserve cached raw-body access and existing rewindable behavior.
• Add one-shot-input unit coverage and integration coverage through the real WEBrick HTTP parser, Rack 3 handler, Rack::Request, and Rack::Lint (no listening socket).
• Cover valid/invalid signatures, repeated cached reads, consumed input, and buffering before the first consumer. Existing StringIO tests protect rewindable behavior.
• Document middleware ordering and Rack::RewindableInput::Middleware as the buffering option.

No CI matrix changes, Rack version-selection scaffolding, or runtime dependency changes. Rackup and WEBrick are test-only dependencies.

Verification

27 Events examples pass with Rack 3.2.7 / rackup 2.3.1. Focused RuboCop passes. Existing local bigdecimal extension warnings remain unrelated.

AI-generated implementation and PR description by GitHub Copilot on behalf of @dblock.

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Coverage Report for CI Build 37315033686

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Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
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Coverage increased (+0.1%) to 91.383%

Details

• Coverage increased (+0.1%) from the base build.
• Patch coverage: 70 of 70 lines across 3 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	6232
Covered Lines:	5695
Line Coverage:	91.38%
Coverage Strength:	104.61 hits per line

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💛 - Coveralls