Honor IndexAttribute on header collection mappings

[justadityaraj]

Problem

With headers enabled, auto-mapping dropped every enumerable member before applying its attributes. A collection declared with [Index(...)] was therefore left unmapped, even though the equivalent explicit ClassMap worked.

Fix

Keep skipping unindexed enumerable members, but retain collections that declare an IndexAttribute. Add a regression test for the reported [Index(2, -1)] case.

User impact

Collection properties can now use index ranges with attribute-based mapping when a CSV has headers, matching explicit class-map behavior.

Verification

• Collection converter tests: 3 passed
• Type-conversion tests: 146 passed; the same 2 Windows newline assertions fail on clean master
• Full net9.0 test suite: 1,062 passed; only those same 2 baseline newline assertions failed
• git diff --check

Fixes #2364