Feat/expose mcps streamable http custom httpx factory parameter

[mikeedjones]

Reopen of https://github.com/google/adk-python/pull/2997 after the accepted changes in that PR were reverted by https://github.com/google/adk-python/commit/e15e19da05ee1b763228467e83f6f73e0eced4b5

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This PR addresses https://github.com/google/adk-python/issues/3005 and https://github.com/google/adk-python/issues/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 https://github.com/google/adk-python/pull/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.

[gemini-code-assist[bot]]

Summary of Changes

Hello @mikeedjones, I'm Gemini Code Assist[^1]! 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 re-implements a previously reverted feature, allowing developers to provide a custom httpx_client_factory when initializing an MCPSessionManager for Streamable HTTP. This change significantly enhances flexibility by enabling fine-grained control over the underlying HTTP client, particularly for managing SSL certificate configurations, thereby resolving critical issues related to secure communication with MCP servers.

Highlights

• Reintroduces httpx_client_factory parameter: This PR reopens a previous change to expose a custom httpx_client_factory when creating an MCPSessionManager for Streamable HTTP connections.
• Enables SSL certificate control: By exposing the httpx_client_factory, users can now control the SSL certificates used for MCP server connections, addressing issues #3005 and #2963.
• Introduces CheckableMcpHttpClientFactory: A new runtime_checkable protocol is added to type-check custom HTTPX client factories, ensuring type safety and adherence to the expected interface.
• Adds comprehensive unit tests: New tests ensure that both custom and default httpx_client_factory instances are correctly utilized by streamablehttp_client, verifying the intended behavior.

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[ryanaiagent]

Hi @mikeedjones, Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[ryanaiagent]

Hi @DeanChensj , can you please review this.