Fix silent key truncation on short BIO_write in key encoders

[sameehj]

PR body

## Summary

The wolfProvider key encoders (RSA, ECC, ECX, DH, ML-DSA) wrote the encoded
key to the output BIO with a single `BIO_write` and treated any positive
return as complete success:

```c
rc = BIO_write(out, keyData, (int)keyLen);
if (rc <= 0) {
    ok = 0;
}

BIO_write is allowed to write fewer bytes than requested (a "short write"),
which returns a positive value smaller than keyLen. The old check only
rejected rc <= 0, so a short write was reported as success while the DER/PEM
output was silently truncated. Application-supplied BIOs (sockets, pipes,
non-blocking or custom sinks) can legitimately short-write, so a caller could
receive and persist a corrupt, incomplete key with no error.

Fix

Add a shared helper wp_write_bio() in src/wp_internal.c that loops until
every byte is written, and use it in all five encoders. The helper mirrors the
existing wp_read_der_bio style (WOLFPROV_ENTER/LEAVE, ok flag, Doxygen
header). Each iteration either advances past the bytes written or fails, so
there is no busy-loop or hang.

Encoder sinks are used synchronously, so a non-positive return is treated as a
hard failure rather than spinning on BIO_should_retry — it fails closed
instead of truncating.

Testing

New deterministic regression test test_ecc_encode_short_write_bio (unit case
143). It builds a custom BIO that accepts exactly one byte per write, encodes a
P-256 key as DER and PEM SubjectPublicKeyInfo through it, and asserts the
captured bytes equal the full reference encoding from OSSL_ENCODER_to_data.
No threads or timing — binary pass/fail.

Before/after (case number: ./test/unit.test --list | grep short_write):

• Test-only commit (fix reverted): ./test/unit.test 143 fails with
  Short write truncated output: 1 of 91 bytes.
• Fix applied: ./test/unit.test 143 passes for DER and PEM.
• Full unit suite: 204/204, 0 failures.

Fenrir

Addresses Fenrir finding 11560 (short-write truncation in key encoders).

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #481

Scan targets checked: wolfprovider-bugs, wolfprovider-src

Findings: 1
1 finding(s) posted as inline comments (see file-level comments below)

This review was generated automatically by Fenrir. Reported findings require changes before merge.

[padelsbach]

Nice fix, thanks

changes addressed