test(db): add SHA-256 known-vector tests

[GaspardKirira]

Summary

Add known-vector and incremental-update tests for the internal SHA-256 implementation used by migration checksums.

Checksum correctness is required to reliably detect modified migration files.

Current behavior

The module includes a self-contained SHA-256 implementation with:

• reset support;
• incremental updates;
• binary updates;
• digest generation;
• hexadecimal encoding;
• the sha256_hex() convenience function.

No automated tests currently validate the implementation.

Expected behavior

The implementation must match standard SHA-256 test vectors and produce identical results regardless of input chunking.

Scope

• Test the empty string.
• Test "abc".
• Test a longer standard vector.
• Test incremental single-byte updates.
• Test updates split into different chunk sizes.
• Test binary input containing zero bytes.
• Test hexadecimal output length.
• Test reset and hasher reuse.
• Preserve the existing public API.

Suggested files

• modules/db/tests/sha256_test.cpp
• modules/db/src/Sha256.cpp

Acceptance criteria

• All expected digests use published SHA-256 values.
• Incremental and one-shot hashing produce identical digests.
• Binary data is handled correctly.
• Reusing a reset hasher produces correct results.
• Hexadecimal output contains exactly 64 lowercase characters.
• Existing public APIs remain unchanged.

Non-goals

This issue does not include:

• replacing SHA-256 with an external crypto library;
• adding other hash algorithms;
• benchmarking hash performance.

Suggested labels

• scope:db
• type:ci
• tests
• good first issue
• status:ready