Cookie unit tests #95

Description

Add unit tests for request-cookie parsing and generated Set-Cookie fields. Main has tests for preserving manually constructed repeated cookies in adapter conversion, but no portable parsing/serialization helper or associated unit suite (exported core modules, raw header extractor, generic response construction).

Steps to reproduce

1. Inspect cookie-related tests and the core API surface.
2. Distinguish adapter tests using literal Set-Cookie strings from parser/builder behavior checks.
3. Look for assertions covering parsed request values and generated cookie attributes.

Expected behavior

Focused portable unit tests verify cookie parsing and Set-Cookie serialization independently of provider runtimes.

Impact

Cookie helper behavior would otherwise lack regression protection beyond raw HTTP header transport.

Proposed fix

Add tests alongside the implementations from #93 and #94. Cover valid/malformed request input, documented duplicate handling, each supported attribute, and multiple generated Set-Cookie values.

Done when

• Request-cookie parsing covers ordinary and malformed input plus the chosen duplicate policy.
• Generated Set-Cookie fields cover each supported attribute and invalid values.
• Multiple cookies are asserted as separate fields; the tests need no provider credentials.

Related

Parent story #93; builder #94; epic #88. Existing transport coverage comes from merged PR #306 and PR #379.