AgentTool SPI + module restructure: agent-plugin tree, agent -> agent-runtime, connector-api under connector-plugin (follow-up to #5408)

[qqeasonchen]

Follow-up to #5408 (discussion in #5407): turn AgentTool into a real repo-standard SPI extension point plus the module restructure to host agent plugins in-tree.

Commit 1 — AgentTool SPI

• @EventMeshSPI on AgentTool + a new AGENT_TOOL entry in EventMeshExtensionType. Implementations register via a META-INF/eventmesh/org.apache.eventmesh.agent.tool.AgentTool service file (mytool=com.example.MyTool) — the same convention as every other EventMesh plugin.
• ToolRegistry.registerSpi(name) — resolves an implementation by SPI name through EventMeshExtensionFactory (singleton semantics, classpath + plugin-jar loaders).
• -Dagent.tools.spi=<name,...> in AgentApplication — enables SPI tools alongside the connector-backed tools from Agent extension points: LlmClient/ConversationMemory SPI, connector-backed agent tools, event triggers #5408.
• plugin/agent/ classpath convention — the agent launcher puts every jar under plugin/agent/ on the classpath (mirrors plugin/connector/ in the connector runtime).

Commit 2 — module restructure (rename detection = pure R100 moves)

• eventmesh-connector-api → eventmesh-connector-plugin/eventmesh-connector-api — the connector SPI jar lives under its plugin family, like storage/protocol.
• eventmesh-agent → eventmesh-agent-runtime — it is a process runtime, matching eventmesh-connector-runtime.
• New eventmesh-agent-plugin/ tree — umbrella + eventmesh-agent-plugin-http-fetch (first in-tree AgentTool plugin: fetch a URL, return the truncated body; pluginType=agentTool, pluginName=http-fetch, service file included). New in-tree tools need zero wiring: dist-agent installs every pluginType==agentTool module into dist-agent/plugin/agent/<name>/.
• All references updated: settings.gradle, root build.gradle (findProject paths + the new plugin install block), all 23 connector plugin build.gradle files, architecture-guard, runtime test dep, docs (agent-tools.md, client-java.md).

User-facing workflows after this PR

# in-tree plugin (shipped): already built into dist-agent/plugin/agent/http-fetch/
AGENT_TOOLS_SPI=http-fetch

# third-party: implement AgentTool, ship the service file, drop the jar into plugin/agent/
AGENT_TOOLS_SPI=risk-check

Tests

• AgentToolSpiTest.registerSpiResolvesThroughTheFactory — resolves echo-test through the real EventMeshExtensionFactory using a test service file + EchoTestTool, invokes it end-to-end
• AgentToolSpiTest.registerSpiRejectsUnknownName — unknown SPI name throws IllegalArgumentException
• Full agent suite: 11/11 green; HttpFetchTool compiles + checkstyle clean
• Moves are pure renames (git R100, zero content diff) — no semantic change to moved code

Notes

• eventmesh-agent-runtime gains an explicit compile dep on eventmesh-spi (the factory module); already transitively present via the SDK.

[qqeasonchen]

Docs pass added (13c5f500): cross-links the agent story end to end —

• README.md / README.zh-CN.md: new LLM streaming agent runtime feature bullet (connector-as-tool, SPI agent plugins, event triggers) + a matching Agent tools & event triggers row in the capability status table (Experimental, links Agent extension points: LlmClient/ConversationMemory SPI, connector-backed agent tools, event triggers #5408/AgentTool SPI + module restructure: agent-plugin tree, agent -> agent-runtime, connector-api under connector-plugin (follow-up to #5408) #5409) + docs-list entries
• docs/feature/a2a.md: new Related capabilities section pairing A2A (agent-to-agent) with the streaming agent runtime (agent-to-LLM), including the typical orchestrator → specialist-with-tools composition
• docs/index.md: A2A entry now points to its agent-tools companion