Add Novita AI as a provider option

[jax-novita]

Summary

• Adds Novita AI as a provider option.

Validation

Compared against the baseline on f3e5e812677f: compared_to_baseline. Pre-existing failures are left as-is.

Reviewer Notes

• This is a docs/config-only change (README provider table + one host-matrix unit test assertion); no new runtime code path was added, so cargo test -p buzz-agent --lib config:: passing (121/121, from the original submission) demonstrates no regression, not that a live request against Novita's endpoint was exercised. This revision was made in an environment without a Rust toolchain, so it was not re-tested; the change is a single string literal swap in a test's input data, and the assertion's expected boolean is unaffected either way (neither URL is an *.openai.com host), but that is an inference, not a verified result.
• Only the config:: unit test module was run (on the original submission), not the full crate, workspace, or desktop (pnpm test/tsc/biome) test suites — the sandbox's Rust toolchain build directory ran out of disk space until CARGO_TARGET_DIR was redirected to a separate volume, so wider verification (integration tests, desktop typecheck) was not attempted, then or in this revision.
• This PR deliberately does not add a Rust Provider enum variant or touch the desktop PERSONA_LLM_PROVIDER_OPTIONS dropdown — Novita AI is exposed only via the existing provider=openai + OPENAI_COMPAT_BASE_URL path, matching the most similar recent precedent (docs(buzz-agent): add OrcaRouter as a supported OpenAI-compatible provider #3096, OrcaRouter). A reviewer may prefer first-class UI treatment instead, or none at all.
• CONTRIBUTING.md requests linking the closest existing PR (docs(buzz-agent): add OrcaRouter as a supported OpenAI-compatible provider #3096, which adds a different third-party OpenAI-compatible provider in an identical two-file shape) and recommends opening an issue first for anything beyond a small fix; this branch does not include a PR description or issue, since that step is left to the maintainer of this submission.
• Six other third-party-provider PRs (feat: add first-class OpenRouter provider support #1975, feat(agent): add Gemini as a first-class LLM provider #2053, feat(desktop): add Together AI as a provider option #3055, feat(buzz-agent): first-class ollama provider alias + local-models docs #3152, docs(buzz-agent): add OrcaRouter as a supported OpenAI-compatible provider #3096, Add goose and Ollama as first-class onboarding providers #2464) have been open with zero review comments for days to weeks, including docs(buzz-agent): add OrcaRouter as a supported OpenAI-compatible provider #3096 which is structurally identical to this change — there's no evidence this category of PR gets prompt attention.
• The base URL literal in the host-matrix test was corrected in this revision. The original commit used https://api.novita.ai/v3/openai, an older path that Novita's current documentation (novita.ai/docs/guides/llm-api) no longer shows; the currently documented endpoint is https://api.novita.ai/openai. Both paths return identical, working results today (verified against /models and /chat/completions), and the test's expected boolean is unchanged either way — so this was a stale citation rather than a functional break, but if your own testing shows Novita's API surface has moved again since 2026-08-01, please treat your own verification as authoritative over this PR's claim.