[Feature Request] Bearer-token HTTP client for runtime invocation and InvokeAgentRuntimeCommand

Start with bedrock_agentcore.runtime.AgentCoreRuntimeClient and its existing _parse_runtime_arn and get_data_plane_endpoint helpers. Review implementation in pull request #422, including its 72 unit tests, then verify the proposed invocation, streaming, command, and session-stop behavior against the stated JSON, SSE, and AWS EventStream responses and the full suite.

Problem

The SDK currently ships bedrock_agentcore.runtime.AgentCoreRuntimeClient, which generates WebSocket URLs and headers (SigV4, SigV4 presigned, and OAuth bearer variants) but does not itself invoke the runtime over HTTP. There is no first-class client in the SDK for:

1. Calling POST /runtimes/{arn}/invocations with a bearer token (JWT/OAuth) and handling both JSON and SSE (text/event-stream) responses.
2. Calling POST /runtimes/{arn}/commands — the InvokeAgentRuntimeCommand API — and parsing its AWS EventStream (application/vnd.amazon.eventstream) response.
3. Calling POST /runtimes/{arn}/stopruntimesession.

Today, callers authenticating via Cognito / OAuth client credentials have to hand-roll urllib3 / requests and a botocore.eventstream.EventStreamBuffer loop, duplicating logic across teams.

Use cases

• Python apps that authenticate end users via Cognito and need to invoke a deployed agent runtime without passing IAM credentials around.
• Automated tests that want to exec shell commands inside an AgentCore container via InvokeAgentRuntimeCommand.
• FastAPI / MCP servers that need to stream tokens from a runtime to a browser (async-for over SSE chunks).

Proposed solution

Extend the existing AgentCoreRuntimeClient with HTTP invocation methods, keeping everything on a single client:

• invoke (blocking, JSON or SSE)
• invoke_streaming (sync generator over SSE chunks)
• invoke_streaming_async (async generator; thread-pumped for use in async frameworks)
• execute_command (blocking, accumulates stdout / stderr / exitCode / status)
• execute_command_streaming (yields parsed EventStream events)
• stop_runtime_session
• AgentRuntimeError exception type

Design constraints:

• Per-call bearer auth, matching the shape of the existing generate_ws_connection_oauth. Each method takes (runtime_arn, bearer_token, ...) so the same client can be reused across rotating credentials.
• Lazy urllib3.PoolManager. The pool is constructed on first HTTP call via a @property, so callers that only use the existing SigV4 URL-generation methods pay zero cost.
• Reuse existing helpers. URL construction uses _parse_runtime_arn (already on the class) and get_data_plane_endpoint from bedrock_agentcore._utils.endpoints.
• urllib3 (already an SDK dep) and botocore.eventstream.EventStreamBuffer (transitive through boto3) — no new top-level dependencies.

Proof of concept

Implementation in #422 with 72 new unit tests, 96% branch coverage on the modified class, and the full upstream suite still passing (1505 / 0 failed).

Alternatives considered

• Users write their own — works today, but the EventStream framing is tricky enough that most implementations I've seen have subtle bugs around chunk boundaries and the chunk envelope.
• Sibling class AgentCoreRuntimeHttpClient — initial approach in an earlier revision of #422. Kept responsibilities cleaner (URL generator vs. network client) but required a second import and discovery point. Reviewer feedback preferred a single-client shape, which the current PR delivers.