feat(a2a): forward RunAgentInput.context and tools through @ag-ui/a2a

Follow-up to #1936 / #1938.

### Context
#1936 fixed `RunAgentInput.state` being dropped by `@ag-ui/a2a` (forwarded via `x-agui-state` metadata + STATE_SNAPSHOT/STATE_DELTA on the inbound side). That PR intentionally scoped to `state` only.

### Remaining gap
`A2AAgent.createSendParams()` still does not forward the other two host-context fields defined on `RunAgentInput`:
- `context?: ContextVariable[]` — semantic prompt items
- `tools?: Tool[]` — extra tools for the run

Both are silently dropped, so an A2A-bridged agent has no access to them.

### Proposed work
- Forward `context` and `tools` through `createSendParams()` using the same metadata-extension pattern #1936 established for `state`.
- Add tests mirroring the #1936 suite (passthrough + empty/omitted cases) for each field.