fix(ecdsa): narrow hex char validation in Signature::from_str

[PrarthanaPurohit]

The Issue:
The original code was validating hex strings with an incorrect pattern:

.all(|&byte| matches!(byte, b'0'..=b'9' | b'a'..=b'z' | b'A'..=b'Z'))

This was checking for:

• Digits: 0-9 ✓
• Lowercase letters: a-z ✗ (should be only a-f)
• Uppercase letters: A-Z ✗ (should be only A-F)

The Problem:
The validation was accepting invalid hexadecimal characters like g-z and G-Z, which would cause parsing errors later when trying to convert the string to a signature.

The Fix:
Changed to the correct hex validation pattern:

.all(|&byte| matches!(byte, b'0'..=b'9' | b'a'..=b'f' | b'A'..=b'F'))

Now it properly validates only valid hexadecimal characters (0-9, a-f, A-F), preventing invalid input from passing the initial check and ensuring the from_str implementation correctly rejects non-hex strings early.

[newpavlov]

Closing in favor of #1255.