[Bug]: SDK ships no native-image reachability metadata, breaking GraalVM builds

### What happened?

I hit six distinct blockers building a GraalVM native image of an application that uses the SDK's Quarkus reference server. Each one was only visible after the previous had been fixed, and none of them produced a useful error message at runtime: the application answers HTTP 500 and logs nothing.

Context: I am building an A2A server extension for Quarkus LangChain4j, which exposes a LangChain4j AI service as an A2A agent. It started from a [stalled PR](https://github.com/quarkiverse/quarkus-langchain4j/pull/1994) by another contributor, who is not involved in this work; that PR predates the 1.x SDK, and most of the current code is my own, bringing it to `1.2.0.Final` and to GraalVM native image, which is a first-class target for Quarkus applications. The goal is to land the extension in quarkus-langchain4j proper, so this SDK would ship inside a released Quarkus extension.

| # | Symptom | Root cause | My workaround |
|---|---|---|---|
| 1 | `Detected a started Thread in the image heap. Thread name: MainEventBusProcessor` | `MainEventBusProcessorInitializer` observes `@Initialized(ApplicationScoped.class)`, which Quarkus fires during static initialization | veto the initializer, start the processor on `StartupEvent` instead |
| 2 | `AgentCard specifies transport interfaces for unavailable transports: [JSONRPC]. Available transports: []` | transports are discovered with `ServiceLoader`, and Quarkus builds native images with `-H:-UseServiceLoaderFeature` | register the providers at build time |
| 3 | every **error** response becomes HTTP 500 | Gson reflects over `ErrorDetail` | register the spec packages for reflection |
| 4 | `Generated message class "org.a2aproject.sdk.grpc.SendMessageRequest" missing method "getTenant"` | protobuf reflects over the generated messages when parsing a request | register the generated messages for reflection |
| 5 | `Generated message class "com.google.protobuf.Struct" missing method "newBuilder"` | the generated messages are built out of protobuf's own well-known types | register `com.google.protobuf` too |
| 6 | `No default configuration value found for: a2a.blocking.agent.timeout.seconds` | `META-INF/a2a-defaults.properties` is not included in the image | register the resource |

Worth noting for #4: this one is **not** gRPC-specific. I initially assumed a request is only parsed into a protobuf message when gRPC is the transport serving it, and made the registration conditional on gRPC being present. That broke every JSON-RPC-only native image, because the JSON-RPC path converts to protobuf internally as well.

### The request

Would you consider shipping native-image reachability metadata (`reachability-metadata.json`, or `reflect-config.json` + `resource-config.json`) in the SDK jars? That would fix #3–#6 for **every** consumer, not just Quarkus ones, since GraalVM picks the metadata up automatically from `META-INF/native-image/`.

Blockers #1 and #2 are Quarkus-specific, but the SDK ships Quarkus reference modules, so they arguably belong to it too. For #1 in particular: starting a thread from an `@Initialized(ApplicationScoped.class)` observer is questionable in any mode, not only native. Quarkus fires that event during static init, which is the one place a live thread must not be created. Observing `StartupEvent` instead would be a one-line change.

### Why consumer-side registration is fragile: a concrete case from this week

Because consumers have to name the SDK's packages themselves, the metadata silently drifts out of sync with the SDK.

Upgrading my extension from `1.1.0.Final` to `1.2.0.Final`, [#987](https://github.com/a2aproject/a2a-java/pull/987) (`refactor!: resolve all split packages`) moved `ErrorDetail`:

```
org.a2aproject.sdk.util → org.a2aproject.sdk.spec.util
```

My reflection registration still named the old package, which no longer exists, so blocker #3 came straight back: **every error response returned HTTP 500 again**. All 41 of my JVM unit tests passed against 1.2.0. The failure only appeared in the native integration tests, and only on the two tests that assert an error response. Had the metadata lived in the SDK jars, it would have moved along with the class and nobody downstream would have noticed.

### Verified against 1.2.0.Final

There is no `reachability-metadata.json`, `reflect-config.json`, `native-image.properties`, `@RegisterForReflection` or GraalVM substitution anywhere under `src/main` in the repository, and blockers #1, #2 and #6 still reproduce as described. `MainEventBusProcessorInitializer` still observes `@Initialized(ApplicationScoped.class)`.

I would be glad to prepare a PR for the metadata, and separately for the `StartupEvent` change, if you agree with the direction, though I would rather confirm the shape you prefer before writing it.

### Relevant log output

```shell
# 1: build time
Error: Detected a started Thread in the image heap. Thread name: MainEventBusProcessor

# 2: startup
java.lang.IllegalStateException: AgentCard specifies transport interfaces for unavailable
transports: [JSONRPC]. Available transports: []

# 4: request time, HTTP 500
Invalid request content: Generated message class
"org.a2aproject.sdk.grpc.SendMessageRequest" missing method "getTenant"

# 5: request time, HTTP 500
Generated message class "com.google.protobuf.Struct" missing method "newBuilder"

# 6: startup
java.util.NoSuchElementException: No default configuration value found for:
a2a.blocking.agent.timeout.seconds
```

### Environment

- a2a-java `1.2.0.Final` (originally found on `1.1.0.Final`)
- Quarkus 3.33.2, Mandrel 25.0.2

### Code of Conduct

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