test(swift-sdk): assert transaction decoder error cases

[thepastaclaw]

Summary

• removes a tautological double-reversal assertion from testTxidDisplayHexMatchesExplorerOrder
• asserts specific PlatformWalletError cases for malformed/trailing transaction bytes and empty input

Validation

• git diff --check
• bash build_ios.sh --target mac --profile dev built DashSDKFFI.xcframework for macOS, then failed the script follow-up iOS Simulator app build because this invocation only produced a macOS library
• swift test --filter TransactionDecoderTests/testMalformedBytesThrowDeserialization passed
• swift test --filter TransactionDecoderTests/testEmptyInputThrowsInvalidParameter passed
• swift test --filter TransactionDecoderTests still fails 4 decode happy-path tests with deserialization("IO error") using the freshly built FFI slice; the two changed error-path tests pass

Addresses CodeRabbit feedback on #3981.

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[thepastaclaw]

✅ Review complete (commit f0b9a25)

[thepastaclaw]

Code Review

Test-only PR that removes a tautological double-reversal assertion and tightens two error-path tests to match specific PlatformWalletError cases (.deserialization and .invalidParameter). All four reviewers independently converged on no in-scope findings; the assertions correctly correspond to the enum in PlatformWalletResult.swift and the documented TransactionDecoder.decode behavior. No consensus-critical or FFI-boundary concerns.

Note: GitHub does not allow me to approve my own PR, so this is posted as a COMMENT while preserving the verified clean review.

Source: reviewers: opus (general), claude-sonnet-5 (general), gpt-5.5[high] (general, failed_or_unparseable), opus (ffi-engineer), claude-sonnet-5 (ffi-engineer), gpt-5.5[high] (ffi-engineer, failed_or_unparseable); verifier: opus; specialists: ffi-engineer