Agent extension points: LlmClient/ConversationMemory SPI, connector-backed agent tools, event triggers

[qqeasonchen]

Fixes #5407

Summary of changes

Agent extension points (connector-first):

• LlmClient SPI (agent.llm): token stream() + function-calling chat() with ToolSpec/ToolCall/Completion records; OpenAiLlmClient implements it and gains non-streaming chat() (OpenAI tools wire format, mock-tested)
• ConversationMemory SPI: ConversationStore (in-memory sliding window) now implements the interface; persistent backends can be constructor-injected
• AgentTool SPI + ToolRegistry: name+schema+invoke tools; StreamingAgent constructor takes an optional registry and, when non-empty, runs a bounded function-calling loop (≤5 iterations, tool results fed back as messages). No tools registered → behavior unchanged token-by-token streaming
• ConnectorToolAdapter: any SinkConnector → write tool (args object wrapped as one CloudEvent → put()+commit()); any SourceConnector → read tool (poll() one batch → JSON array). Class-based (system-property wired in AgentApplication) and instance-based overloads
• Event-driven triggers: agent.subscribe.topics / agent.trigger.output.topic — each consumed CloudEvent becomes a prompt (trigger:<eventId> conversation), answered with tools, answer published to the output topic for sink delivery. Uses a dedicated CloudEventsClient so the agent lite-channel poller is untouched
• Port drift fix: AgentApplication, ConnectorApplication, start-connector.sh, connector.properties, agent.properties, StreamingCallDemo — default runtime URL 8080 → 10105 (matches EventMeshApplication's default)
• Docs: new docs/feature/agent-tools.md (extension surface table, connector-as-tools, triggers, embedder API, config reference) + docs/index.md entry

Agent boot wiring (AgentApplication): agent.tools.sink.<name>=<fqcn> / agent.tools.source.<name>=<fqcn> + agent.tools.props.<name>.* build the tool registry; connector jars just need to be on the agent classpath.

Tests

• ToolRegistryTest — registry lookup, spec rendering, unknown-tool error (3 tests)
• ConnectorToolAdapterTest — sink tool delivers args as CloudEvent (asserts serialized data), source tool returns batch as JSON array + empty batch (3 tests)
• OpenAiLlmClientChatTest — tool-call parsing, plain-text completion, tools advertised in request body (3 tests, in-process mock)
• All 9 green via JUnit launcher (hermetic; no external LLM)
• ConversationStoreTest unchanged and still passing (interface extraction is source-compatible: ConversationStore still concrete)

Notes for reviewers

• Record components ToolSpec/ToolCall are flagged [WARN] by local checkstyle 9.0 (MethodName on record decls); records are used across the codebase (e.g. AgentControlClient.RegisterResult) so CI's checker accepts them
• eventmesh-agent now depends on eventmesh-connector-api (compile) + io.cloudevents:cloudevents-core — the architecture guard only constrains plugin→runtime, this direction is unconstrained
• Tool-loop answers are published as a single chunk (per-token streaming of the final answer is a follow-up noted in the issue)

[qqeasonchen]

Rebased onto the latest develop (2d1dc1a13) to resolve the conflict with #5406's agent hardening.

Conflict resolution notes:

• AgentApplication — kept develop's [Feature] A2A gateway production wiring: main-process boot, RocksDB task store, auth, metrics, contextId #5405/[ISSUE #5405] A2A gateway production wiring: main-process boot, RocksDB task store, auth, metrics, contextId #5406 hardening (LLM key fail-fast, heartbeat fail-limit exit, maxConversations LRU store) and layered the connector-tools registry + event-trigger wiring on top; runtime-url default fix from this PR was redundant for the agent module (develop already moved it to 10105) and is dropped in favor of develop's version
• ConversationStore — develop's LRU-bounded version is the new base; this PR's change is now just implements ConversationMemory + @Override markers (both constructors preserved)
• agent.properties — develop's [Feature] A2A gateway production wiring: main-process boot, RocksDB task store, auth, metrics, contextId #5405 keys kept; this PR's tool/trigger keys doc appended
• Connector-runtime / examples port drift (8080 → 10105) is still needed on develop and remains part of this PR
• Full javac + 9/9 unit tests re-verified against the merged sources