feat(a2a): wire A2A Gateway onto Runtime via TaskStore (issue #5302 Sub-PR D1) (#5313)

* feat(a2a): wire A2A Gateway onto Runtime via TaskStore (issue #5302 Sub-PR D1)

Brings the A2A Gateway back into the Runtime, this time with the unified
control-plane stores from #5301 Sub-PR A/C as the durable backend instead
of the in-memory TaskRegistry that PR #5260 introduced.

What lands
----------
* A2AGatewayService — task lifecycle controller. Replaces the in-memory
  TaskRegistry with TaskStore (Sub-PR A/C). A small runtime cache holds
  the two fields TaskStore does not model: parentTaskId (used to render
  child-task queries) and taskEpoch (the per-task counter the store
  requires for stale-write rejection on updateStatus). Both caches are
  rebuildable from the store on a fresh JVM; the cache keys are
  taskIds, so the cache size tracks active tasks only.
* A2AGatewayHttpHandler — Netty HTTP handler for the REST + SSE API
  (/a2a/tasks, /a2a/tasks/{id}, /a2a/tasks/{id}/stream, /a2a/health).
  SSE registers a StatusSubscriber so intermediate PENDING -> RUNNING ->
  COMPLETED transitions stream to the client.
* A2AGatewayServer — Netty HTTP bootstrap. Production wires
  EventMeshA2ATransport (Runtime-bridged) for the A2AMessageTransport
  dependency; tests pass an in-process transport. The weather-agent
  demo from PR #5260 is removed — the demo client (A2AGatewayDemo) and
  the Meta-ized AgentCard registry land in Sub-PR D2.
* AgentCardRegistry + InMemoryAgentCardRegistry — minimal discovery
  surface (isAgentRegistered / registerCard / getCard). A Meta-backed
  implementation backed by SessionStore (Sub-PR A) lands in D2.

Status mapping (PR #5260 -> Sub-PR A/C)
---------------------------------------
  SUBMITTED -> PENDING, WORKING -> RUNNING, CANCELLED -> CANCELED (one L)
The legacy wire vocabulary stays available via A2AGatewayService.TaskState
and the toLegacyState() helper. JSON response payloads still emit
SUBMITTED/WORKING/CANCELLED so existing A2A clients do not break.

Documentation
-------------
* docs/a2a-protocol/README.md carries an EXPERIMENTAL banner that lists
  the three pieces still missing for production (TaskExpirer, Meta-ized
  AgentCard, Testcontainers E2E) — all scheduled for Sub-PR D2.

Acceptance against issue #5302
------------------------------
* "A2A transport over the Runtime delivery path" — A2AGatewayService
  takes an A2AMessageTransport; production wires EventMeshA2ATransport
  which already bridges A2A onto UniIngressService (issue #5301 Sub-PR
  A's runtime ingress). The parallel InMemoryA2AMessageTransport is no
  longer required by the gateway path.
* "A2A tasks persist through TaskStore and recover across restarts" —
  createTask / updateStatus / getTask all go through the persistent
  store. On a fresh JVM, a gateway that had pending tasks sees them
  with their last persisted status; in-flight work that was awaiting
  a response will be picked up on the next status callback.

Tests (7, all passing)
----------------------
* A2AGatewayServiceTest — 6 tests: create+complete, cancel, cancel-on-
  unknown, submit-to-unregistered, parent-child index, task-not-found.
  Uses an in-process TaskStore (mirrors Sub-PR A's test stub) and an
  in-process A2AMessageTransport that handles A2A's + single-segment
  wildcards.
* A2AGatewaySmokeTest — Netty HTTP /a2a/health loopback check.

Not in D1 (Sub-PR D2)
---------------------
* TaskExpirer reaper (periodic TaskStore.expireStale sweep).
* Meta-backed AgentCardRegistry (SessionStore).
* Testcontainers fault-injection E2E.

* fix(a2a): address checkstyle violations in A2A Gateway D1 (PR #5313)

A2AGatewayHttpHandler.java:
- Remove unused imports (A2AProtocolConstants, A2ATopicFactory, TaskState)
- Expand try-catch blocks for limit/offset parsing (EmptyCatchBlock, NeedBraces)
- Expand handleList loop control flow (NeedBraces)
- Collapse duplicate blank line between package and imports (EmptyLineSeparator)

A2AGatewayServiceTest.java:
- Expand single-line if-blocks in InProcessTaskStore.listByAgent
- Expand if-block in InProcessTransport.matches
- Expand if-blocks in tearDown
- Move taskId declaration closer to its use (VariableDeclarationUsageDistance)

A2AGatewaySmokeTest.java:
- Rewrite StubTaskStore / NoopTransport with multi-line method bodies
  (LeftCurly, RightCurlyAlone, NeedBraces, EmptyLineSeparator)
- Add missing static imports (assertEquals, assertNotNull)

Build PR #5313 fix.

* fix(a2a): test imports + variable usage distance (PR #5313 followup)

A2AGatewayServiceTest.java:
- Fix ImportOrder: AgentCapabilities before AgentCard (alphabetical)
- Fix VariableDeclarationUsageDistance for f: inline taskId into
  submitTask call so the declaration is closer to first use

A2AGatewaySmokeTest.java:
- Fix ImportOrder: static Assertions imports before non-static
  TaskStore import

Build PR #5313 followup.

* fix(a2a): correct cancelMarksTaskCanceled test body (PR #5313 followup)

The previous fixup broke the test logic by calling TaskResult.getTaskId()
which does not exist. Restore the original taskId declaration pattern
while keeping the f declaration within 3 lines of its first use:

- Declare taskId first
- Declare f immediately after
- Inline cancelTask result into the assertTrue (no intermediate boolean)
- Move f.get() call right after the cancel assertion so f's first use
  is within 3 lines of its declaration

Build PR #5313 followup.