Validate hex input strictly

[sc0Vuai]

decode(), Str::encode(…, 'hex') and Str::decodeHex() accepted malformed hex:

• Any input containing a single hex digit passed validation, and invalid characters were silently dropped: decode("0x8zz1") → "8z", encode("0xzz") → "zz".
• Every 0x in the string was removed, not just the prefix, so 0x0x12 was accepted.
• decode() padded odd-length input: 0x8 → 08.

Now only a leading 0x is stripped and the rest must be hex digits; a trailing newline is also rejected. decode() rejects odd-length input. Encoding still pads odd-length hex ("0x0" → "00"), and uppercase digits are still accepted.

Behavior change: these inputs now throw InvalidArgumentException. That includes encode("0xhello"), which used to produce invalid output (830hello).

Adds regression vectors for invalid and still-valid hex input.

🤖 Generated with Claude Code (https://claude.com/claude-code)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.44%. Comparing base (a22b1ba) to head (4114d21).

Additional details and impacted files

@@              Coverage Diff              @@
##             master      #33       +/-   ##
=============================================
+ Coverage     84.27%   94.44%   +10.16%     
- Complexity       59       61        +2     
=============================================
  Files             3        3               
  Lines           159      162        +3     
=============================================
+ Hits            134      153       +19     
+ Misses           25        9       -16     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.