[Order 05][P2] Harden duplicate detection and null binary materialization

[AGiorgetti]

Priority order

Order: 05 of 09
Priority: P2 correctness

Problem

Provider error handling and binary materialization have fragile edge cases.

Evidence:

• src/NEventStore.Persistence.Sql/SqlDialects/MySqlDialect.cs:43
• MySQL duplicate detection reflects a Number property and immediately casts it.
• src/NEventStore.Persistence.Sql/CommitExtensions.cs:87
• GetByteArray returns [default] for null/DBNull.Value.

Impact

• MySQL can mask non-provider exceptions with NullReferenceException or invalid casts during error handling.
• Nullable Headers values can be deserialized from one zero byte instead of returning default, which can fail for legacy rows or manually inserted rows.

Mandatory test-first workflow

• Add tests first to reproduce or verify each behavior.
• Add a unit test proving MySqlDialect.IsDuplicate(new Exception(...)) returns false and does not throw.
• Add a materialization test for a commit row with Headers = NULL.
• Do not change production code until the tests capture the expected behavior.

Proposed action

• Guard MySQL IsDuplicate for missing or non-int Number.
• Prefer provider-specific exception checks where available.
• Return [] for null and DBNull.Value in GetByteArray.

Acceptance criteria

• Duplicate detection does not throw for unrelated exception types.
• Null headers materialize as default/empty headers behavior without deserialization failure.
• Existing duplicate-commit behavior remains covered.

Source: docs/Project-Analysis-Issue-Drafts.md.