Add Responses API support for OpenRouter provider

[chtushar]

Description:

• Added endpoint mappings for OpenRouter Responses API: POST /v1/responses, GET /v1/responses/:id, DELETE /v1/responses/:id, GET /v1/responses/:id/input_items
• Reuses open-ai-base infrastructure for request parameter mapping and response transformation
• Supports OpenRouter-specific parameters: provider routing, plugins, top_k, frequency_penalty, presence_penalty, session_id, and trace
• Both streaming and non-streaming modes are supported via native SSE passthrough and JSON-to-stream conversion for cache hits

Tests Run/Test cases added:

• Type-checked with TypeScript compiler (no new errors introduced)
• Build succeeds with Rollup
• All existing gateway unit tests pass (47/47)

Type of Change:

• New feature (non-breaking change which adds functionality)

Fixes
#1527