Conditional request support (If-Match, If-Unmodified-Since)

[bjaus]

Framework-level support for conditional requests beyond the existing ETag middleware. Enables optimistic concurrency for write operations.

Scope:

• If-Match / If-None-Match header handling for PUT/PATCH/DELETE
• If-Modified-Since / If-Unmodified-Since header handling
• Return 412 Precondition Failed when conditions aren't met
• OpenAPI spec: document conditional request headers as parameters