Inspector backend incompatible with a2a-sdk v1.0 (protobuf API changes)

## Problem
The inspector backend (`backend/app.py`) uses v0.3 SDK types (`FilePart`, `TextPart`,
`ClientEvent`) that no longer exist in a2a-sdk v1.0+, which switched to protobuf types.

When the inspector's `pyproject.toml` requires `>=1.0.0`, the backend crashes on import.
When pinned to `0.3.x`, the SDK's transport layer (`a2a/client/transports/jsonrpc.py:181`)
validates SSE events against v0.3 `SendStreamingMessageResponse` Pydantic models,
which reject v1.0 format responses (`{"task": {...}}` wrapper, no `final` field).

## Impact
Agents implementing A2A spec v1.0 cannot be tested with the inspector.

## Evidence
- TCK at 72.4% confirms the agent is v1.0 compliant
- Inspector fails with 14 Pydantic validation errors on every streaming call

## Suggested fix
Update `backend/app.py` to use v1.0 SDK APIs and update the transport
validation to use v1.0 `StreamResponse` protobuf types.

The issue is in `backend/app.py`. First, examine the current imports and usage of `FilePart`, `TextPart`, `ClientEvent`. Then, review the a2a-sdk v1.0 protobuf API documentation to understand the new types like `StreamResponse`. Update the imports and usage in `backend/app.py`. Also, check the transport validation in `a2a/client/transports/jsonrpc.py` around line 181 to update from `SendStreamingMessageResponse` to the new v1.0 validation. Run the inspector to ensure it no longer crashes and can test v1.0 agents.

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