Broaden provider streaming conformance

[asim]

Goal: keep chat, agent RPC, and A2A streaming reliable across providers instead of only the already-covered paths.

Scope:

• Add or extend provider-gated streaming conformance for ai.Stream where provider keys are available.
• Cover at least one tool-using/agent-chat scenario that asserts streamed final output and provider capability reporting.
• Keep tests skipped when the relevant provider key is absent, so local no-secret CI remains usable.

Acceptance criteria:

• A CI-verifiable command exercises streaming for more than the current provider-specific happy path when keys are present.
• Provider capability docs or fixtures stay in sync with the implemented streaming support.
• Chat/A2A streaming regressions are caught by a focused test or harness target.