server: include body receipt in request deadlines

[jeet-dekivadia]

Summary

• anchor buffered request deadlines before body collection so one absolute budget covers receipt and handler execution
• apply the same bounded collection to error-path drains while leaving client-streaming and bidi request flow unchanged
• document the request-receipt guarantee and add paused-time regressions for stalled unary and server-streaming bodies

Fixes #136

Validation

• git diff --check
• cargo +nightly fmt --all -- --check
• cargo check --workspace --all-features --all-targets
• cargo test --workspace --all-features
• cargo clippy --workspace --all-features --all-targets -- -D warnings

[github-actions]

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