feat(tools): expose httpx_client_factory on RestApiTool and OpenAPITo…

[darshil3011]

…olset

Mirror the pattern merged for MCP in #2997 (StreamableHTTPConnectionParams. httpx_client_factory) on the OpenAPI tool surface.

Adds an optional httpx_client_factory parameter to:

• RestApiTool.init
• RestApiTool.from_parsed_operation
• OpenAPIToolset.init

OpenAPIToolset forwards the factory to every generated RestApiTool the same way ssl_verify and header_provider are already forwarded. When provided, the factory's client is used to issue each API call; when None (default), the existing httpx.AsyncClient(verify=..., timeout=None) construction is preserved exactly. This unlocks httpx.AsyncClient knobs that the narrower ssl_verify parameter can't reach: proxies, HTTP/2, custom transports (e.g. request signing), and shared connection pools.

Closes #5681

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