Feat/expose mcps streamable http custom httpx factory parameter

[mikeedjones]

This PR addresses #3005 and #2963 to allow control over the ssl cert used when connecting to an mcp server by exposing the httpx_client_factory parameter exposed when creating a MCPSessionManager in adk. Overlaps with #2966 but I don't believe that that PR's implementation will work. streamablehttp_client needs a client factory, not a client.

testing plan

Adds test checking that streamablehttp_client uses the custom httpx factory. Could also test that a factory which obeys the McpHttpClientFactory protocol produces valid behavior when the session is opened?

related issues

#2227 and #2881 both request slightly different options to control the ssl certs used internally by adk. I think exposing a httpx factory is a good pattern which could be followed for those issues too.

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[gemini-code-assist]

Summary of Changes

Hello @mikeedjones, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant enhancement to the MCPSessionManager by enabling the injection of a custom httpx_client_factory. This allows developers to fully customize the underlying HTTP client used for interactions with MCP servers, providing flexibility for advanced configurations such as specific SSL certificate management or proxy settings, thereby addressing existing requests for more control over internal ADK HTTP client behavior.

Highlights

• Custom HTTP Client Factory Exposure: The MCPSessionManager now exposes an httpx_client_factory parameter, allowing users to provide a custom factory for creating httpx clients.
• Enhanced HTTP Client Control: This change provides more granular control over the HTTP client used for connecting to MCP servers, addressing needs like custom SSL certificate handling or other advanced HTTP client configurations.
• New Test Coverage: A new test case has been added to ensure that the custom httpx_client_factory is correctly invoked when initializing an MCPSessionManager.

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[adk-bot]

Response from ADK Triaging Agent

Hello @mikeedjones, thank you for your contribution!

To help our reviewers, could you please add a testing plan section to your PR description? This section should briefly describe how you've tested the changes.

You can find more details in our contribution guidelines. Thanks!

[gemini-code-assist]

Code Review

This pull request successfully exposes the httpx_client_factory parameter for StreamableHTTPConnectionParams, allowing for greater control over the internal httpx client. The implementation is clean and the inclusion of a test case is appreciated. I've provided a couple of minor suggestions to improve code style and align the new test with unit testing best practices.