Add public constructors for SizeError and ValidityError

[barunaniket]

Summary

Callers who know, out-of-band, that a conversion would fail due to incorrect source size or invalid source data currently have no way to construct a SizeError or ValidityError directly, and must instead perform a contrived failing conversion just to obtain one (see #3666).

This makes SizeError::new and ValidityError::new public so these errors can be constructed directly and forwarded via the existing From impls into ConvertError, TryReadError, etc.

AlignmentError is intentionally left as-is: its constructor is unsafe because of a soundness invariant (Dst's alignment requirement must be greater than one, relied on elsewhere via unreachable_unchecked), so a safe public constructor would need a checked design (e.g. panicking or Option-returning) rather than a simple visibility change — that seems like a separate decision for maintainers to weigh in on.

Fixes #3666

Test plan

• cargo test --doc — new doc examples for both constructors compile and pass
• cargo test --lib — full library test suite passes (112 tests)
• cargo test --doc (full suite) — 128 doc tests pass
• cargo clippy — no new warnings in error.rs
• cargo build --no-default-features — no_std build succeeds
• rustfmt +nightly --check on the changed file — no diff introduced by this change

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.96%. Comparing base (218f771) to head (ad8d743).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3678      +/-   ##
==========================================
+ Coverage   91.90%   91.96%   +0.05%     
==========================================
  Files          20       20              
  Lines        6175     6183       +8     
==========================================
+ Hits         5675     5686      +11     
+ Misses        500      497       -3     

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[barunaniket]

@joshlf This is ready for review whenever you get a chance — CI is green. Let me know if you'd like any changes.