Fix AttributeError when port is already in use

[dcabib]

Issue

Fixes #7244

Problem

When SAM CLI attempts to bind to a port that's already in use, users see a cryptic AttributeError: 'str' object has no attribute 'decode' instead of a helpful error message about which port is occupied.

This occurs because:

1. In docker-py >= 7.0, APIError.explanation is a str (not bytes)
2. The code calls .decode() on it, causing AttributeError
3. Case-sensitive check ("Ports") misses lowercase variations ("ports")

Solution

• Added safe_decode_docker_message() function that handles both bytes (docker-py < 7.0) and str (docker-py >= 7.0)
• Updated error detection to use case-insensitive check (.lower())
• Added 8 comprehensive unit tests covering all scenarios

Testing

• ✅ All 6931 existing tests pass
• ✅ Added 8 new tests (100% passing)
• ✅ Coverage maintained at 94.04%
• ✅ Lint and formatting checks pass
• ✅ Tested with samdev local invoke on occupied port

Result

Users now see the actual port error message:

Error: ports are not available: exposing port TCP 127.0.0.1:5005 -> 127.0.0.1:0: listen tcp4 127.0.0.1:5005: bind: address already in use

Instead of:

AttributeError: 'str' object has no attribute 'decode'

Checklist

• Code builds correctly
• New/updated tests pass
• All tests pass (make test)
• Coverage maintained at 94%+
• Lint checks pass (make lint)
• Formatting is correct (make black-check)
• Issue linked in commit and PR description
• Added comprehensive unit tests
• Backwards compatible (no breaking changes)