fix(chat): emit one earned terminal per turn

[Floating-Y]

Addresses #857

This PR follows the direction proposed in #857: it uses eventsource-parser for client SSE framing and consolidates server terminal emission at the stream boundary.

The intent is to make done depend on a saved assistant message and to return one terminal event for each stream. I'd appreciate feedback on whether this matches the intended terminal contract, especially for fallback and reconnect paths.

Changes

• Use eventsource-parser for client SSE framing.
• Emit a terminal event when the assistant message is saved or a failure is known, with server-side handling to prevent duplicate terminals.
• Reject empty fallback answers and report unexpected stream closure.
• Add regression coverage for terminal events and SSE framing.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo build --workspace
• cargo test -p utopia-server api::chat -- --test-threads=1 — 48 passed
• cargo test --workspace --quiet -- --skip gbk_euc_h_pdf_extracts_chinese_text — passed
• pnpm -C web test — 135 passed
• pnpm -C web build

The skipped PDF test requires Poppler, which is unavailable on this Windows host; CI installs it.

[WaylandYang]

The ProducerEvent split is the right shape for #857's step 3: every producer exit path I traced (size limit, model error, cancelled-then-answer, fallback, empty answer, save failure, success) now yields an outcome and the spawn loop emits exactly one terminal, with Closed-without-terminal correctly surfaced as an error given how live::emit freezes the snapshot. Two changes before merge:

1. Drop commit b8fec3d2 (vector index drop/build lock in crates/utopia-store/src/vector_index.rs and its test) into its own PR. It is unrelated to #857 and undocumented here; it looks reasonable on its own and deserves its own description.
2. The terminal table in chat_terminal_tests.rs lost its pending list and the injection-point section without gaining rows. The targeted tests do exist on dev (chat_persistence_tests.rs, chat_fallback_tests.rs, chat_registry_tests.rs), so either add the rows or name those tests in the comment so the table stays the place reviewers look.

Optional: make Progress a type that cannot hold a terminal event (today a done frame sent as progress is only caught at runtime), and put the saved message id on done so a client can verify the "earned" claim. The parser swap and its CR/CRLF tests look good.

[WaylandYang]

Applied the two review points as maintainer edits so this can land; your commit and sign-off are kept, mine is added.

1. b8fec3d (vector index drop takes the build lock) is now its own PR, Take the build lock when dropping a vector index so a drop cannot deadlock a concurrent build #908, carried there as your commit with your sign-off. This branch is the chat change alone, rebased onto current dev.
2. The terminal table's header again says where the paths that need fixture-level injection live: chat_persistence_tests.rs, chat_fallback_tests.rs and chat_registry_tests.rs, so the table stays the place a reviewer looks first.

Locally on a fresh database: the 48 chat tests pass, clippy is clean, and the web suite (135 tests) and build pass with the pinned eventsource-parser. Merging once CI agrees. Thanks for the ProducerEvent split; it is the right shape for #857.

[WaylandYang]

Both review points applied: the vector-index commit is #908 on its own, and the terminal table names the targeted test files; 48 chat tests, clippy, and the web suite pass locally; CI green. Approving to clear the earlier request-changes.