a2a: switch synchronous a2a_send to SSE streaming (tasks/sendSubscribe)

[pbranchu]

Summary

Switches the synchronous `a2a_send` tool from the blocking `tasks/send` JSON-RPC method to SSE-streaming `tasks/sendSubscribe`. Adds a shared `consume_sse_stream` helper used by both this PR and the forthcoming async dispatch PR.

This is the foundation PR in a three-PR series that splits the work from the closed #1066 into reviewable chunks:

1. This PR — SSE streaming for the existing sync tool (no kernel changes)
2. `kernel-context-threading` — refactor `KernelHandle::send_message` to plumb `ChannelCallbackContext` as a parameter (eliminates a cross-user race in any future callback machinery)
3. `a2a-async-dispatch` — async tools (`a2a_send_async`, `a2a_check_task`, `a2a_cancel_task`) built on top of PRs 1 + 2

Changes

• New shared helper: `process_sse_line` / `parse_sse_content` / `consume_sse_stream` in `a2a.rs` — single SSE-parsing implementation used by all streaming paths
• `send_task_streaming` — replaces `send_task` for the `a2a_send` tool. Streams bytes from the wire incrementally (does not buffer the full response into a String first)
• UTF-8-safe byte accumulation: bytes accumulate in a `Vec`, only complete newline-terminated prefixes are decoded with `from_utf8_lossy` (avoids panics on chunks that split multi-byte codepoints)
• Wall-clock deadline of 300 s via `tokio::time::timeout` wrapping `consume_sse_stream` (matches the tool description; `reqwest`'s built-in `.timeout()` is per-read, not total-elapsed)
• Tool description for `a2a_send` updated to honestly say "blocks until complete, up to 300 s"
• Cargo.lock: bumps `lettre` to 0.11.22 (RUSTSEC-2026-0141)

Tests

7 SSE parser unit tests covering: normal completion, mid-stream disconnect, malformed JSON, missing final event, concatenated lines, error events, empty stream.

946 runtime + 503 channels tests pass on two consecutive runs (no flakes). `cargo clippy --workspace --tests --all-targets -- -D warnings` clean.

Context

This PR is what closed PR #1066 should have been on its own: a small, focused, self-contained change. The async dispatch tools and kernel refactor are split into their own PRs to keep each one reviewable.

Test plan

• CI passes
• Local: `cargo test -p openfang-runtime --lib -- --skip process_manager --skip subprocess_sandbox` passes
• Confirmed against a mock SSE server in production deployment