Fix parser corruption on unknown channel request types

[jeffellin]

Fixes #221

Summary

This PR fixes the critical parser state corruption bug when NIOSSH receives SSH channel requests with unknown request types. See #221 for detailed problem analysis and reproduction steps.

Changes

• readChannelRequestMessage(): Consume remaining bytes with moveReaderIndex(to: writerIndex) before setting type = .unknown
• Add testUnknownChannelRequestWithPayload to verify bytes are consumed
• Add testParserContinuesAfterUnknownChannelRequest to verify parser state remains valid

This matches the existing pattern in readGlobalRequestMessage() (lines 814-823) which correctly consumes payload bytes for unknown global requests.

Root Cause

When the parser encountered an unknown channel request type (e.g., vendor-specific extensions like session-id@example.com), it would set type = .unknown but fail to consume the payload bytes from the buffer. The unconsumed bytes would corrupt the next packet read, causing CryptoKitError.authenticationFailure on all subsequent SSH messages and making the connection permanently unusable.

Solution

Apply the same buffer-consumption pattern used for unknown global requests:

default:
    // Unknown channel request type - consume remaining bytes to prevent
    // parser state corruption. Without this, leftover bytes cause
    // invalidPacketFormat errors that corrupt the encryption state.
    self.moveReaderIndex(to: self.writerIndex)
    type = .unknown

Test Results

Before fix:

• testUnknownChannelRequestWithPayload: ❌ FAILS - 24 unconsumed bytes in buffer
• testParserContinuesAfterUnknownChannelRequest: ❌ FAILS - throws unknownType(0) error

After fix:

• testUnknownChannelRequestWithPayload: ✅ PASSES - all bytes consumed
• testParserContinuesAfterUnknownChannelRequest: ✅ PASSES - parser continues normally

Full test suite:

$ swift test
Executed 322 tests, with 0 failures

$ swift test \
  --explicit-target-dependency-import-check error \
  -Xswiftc -require-explicit-sendable \
  -Xswiftc -warnings-as-errors
Executed 322 tests, with 0 failures

Breaking Changes

None. This is a pure bug fix:

• Still returns SSHMessage.channelRequest with type = .unknown
• No API changes
• No behavior changes for known request types
• Simply fixes buffer corruption that made connections unusable

Impact

This fix enables NIOSSH to interoperate with:

• Enterprise SSH servers with session tracking extensions
• Cloud provider managed SSH (AWS Session Manager, Azure Bastion, GCP IAP)
• Security appliances with audit/recording extensions
• GitHub Enterprise with custom authentication extensions
• Any SSH server using RFC 4250 name@domain extension format

Checklist

• Tests added and all passing
• Commit message follows template
• No breaking changes
• Matches existing code patterns
• RFC 4250 compliant

[ankushkushwaha]

Suggested change

	// Write a known message after
	// Write a known message afterward