Resolve memory overflow when b256::TryFrom<Bytes> is not 32 bytes

[bitzoic]

Description

Currently, when calling the TryFrom implementation for b256 from Bytes, if the Bytes has less than 32 bytes a memory overflow error will occur. If there is less than 32 bytes, None is now returned.

Example:

    let mut bytes = Bytes::with_capacity(32);
    let mut i = 0;
    while i < 32 {
        // 0x33 is 51 in decimal
        bytes.push(51u8);
        i += 1;
    }
    let res = b256::try_from(bytes);
    assert(res.unrwap() == 0x3333333333333333333333333333333333333333333333333333333333333333);

Closes #6129

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
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• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
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[IGI-111]

I don't think left padding is the correct behavior. Converting less than 32 bytes to a b256 should fail.

You may argue this for u256 though I still think it's dubious to assume user intent there. But for b256 it's categorically erroneous.

[IGI-111]

LGTM but can you update the title and PR message to reflect the changes?