fix(llm): add HTTP-status and transport-level retry to Ollama backend

[bug-ops]

Summary

• Ollama's chat/chat_stream/chat_with_tools sent every request through ollama-rs with no retry, so a single 429/503 or transient connection failure failed the whole turn immediately, unlike Claude/OpenAI/Gemini.
• ollama-rs discards HTTP status codes on non-2xx responses, so ollama.rs now posts chat requests directly via a dedicated reqwest client, reusing ChatMessageRequest/ChatMessageResponse only for serialization, letting retry::send_with_retry retry on real HTTP 429/503 with Retry-After support — the same helper Claude/OpenAI/Gemini use.
• A second layer, send_with_transport_retry, retries transient transport failures (timeout, mid-request connection reset) that never reach send_with_retry's status inspection. Connect-phase failures stay unretried and keep RouterProvider's fallback diagnostics (RouterProvider::chat and chat_stream discard last provider error on fallback exhaustion #5821) intact.
• Also fixes a UTF-8 chunk-boundary data-loss bug in the streaming NDJSON parser this change introduces: it now buffers raw bytes across chunks and decodes only once a complete line is assembled, instead of decoding each chunk independently and silently dropping one that splits a multi-byte character.

Closes #6491

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins
• RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• LLM Serialization Gate: ran live against a real local Ollama instance (0.32.1, qwen2.5:7b), including tool-call round trips; all previously-ignored integration tests plus a new chat_with_tools integration test pass
• New unit tests: HTTP 429/503+Retry-After retry through the real provider methods (wiremock), transport-level retry, UTF-8 chunk-boundary NDJSON buffering, stream-flag JSON body correctness, chat_with_tools retry