feat(adk): support custom CA for non-openai model providers

[supreme-gg-gg]

Currently, only OpenAI BYO models support custom CA. This PR extends the existing API to all non-Vertex-AI model providers (Claude API, Gemini API, Bedrock, Ollama) for both python and go runtimes. This reuses most of existing plumbings and refactors some TLS related code. It also adds a GeminiLm wrapper class in order to support custom CA and custom headers (close #1714 as well).

[Copilot]

Pull request overview

This PR extends non-Vertex provider transport configuration so custom CA handling (and Gemini custom headers) flow through both the Python ADK runtime and the Go ADK runtime. It refactors shared TLS plumbing, adds a dedicated Gemini wrapper on the Python side, and updates tests around the new transport behavior.

Changes:

• Added shared Python TLS transport helpers and applied them to OpenAI, Anthropic, Ollama, Bedrock, SAP AI Core, and a new Gemini wrapper.
• Switched Python model creation for gemini to return KAgentGeminiLlm, enabling custom headers/TLS options instead of returning the raw model name.
• Updated Go Gemini and SAP AI Core model creation to use shared HTTP client construction for TLS/header support.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
python/packages/kagent-adk/tests/unittests/models/test_tls_integration.py	Updates TLS integration tests and adds Gemini transport assertions.
python/packages/kagent-adk/tests/unittests/models/test_openai.py	Adjusts OpenAI TLS mocks to patch the shared SSL helper.
python/packages/kagent-adk/src/kagent/adk/types.py	Refactors shared transport kwarg handling and routes Gemini configs to the new wrapper.
python/packages/kagent-adk/src/kagent/adk/models/_ssl.py	Introduces a reusable TLS mixin for model wrappers.
python/packages/kagent-adk/src/kagent/adk/models/_sap_ai_core.py	Reuses shared TLS client creation for SAP AI Core requests and token fetches.
python/packages/kagent-adk/src/kagent/adk/models/_openai.py	Replaces OpenAI-specific TLS code with the shared mixin.
python/packages/kagent-adk/src/kagent/adk/models/_ollama.py	Adds TLS support to the Ollama wrapper and factory.
python/packages/kagent-adk/src/kagent/adk/models/_gemini.py	Adds a new Gemini wrapper with custom headers and TLS-aware HTTP options.
python/packages/kagent-adk/src/kagent/adk/models/_bedrock.py	Threads TLS settings into Bedrock client creation.
python/packages/kagent-adk/src/kagent/adk/models/_anthropic.py	Adds TLS-aware HTTP client creation for Anthropic.
python/packages/kagent-adk/src/kagent/adk/models/__init__.py	Exports the new Gemini wrapper.
go/adk/pkg/models/sapaicore.go	Switches SAP AI Core to shared HTTP client construction.
go/adk/pkg/agent/agent.go	Wires shared transport config into Gemini and SAP AI Core model creation.

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