Align with modernized TS SDK: hosted default, OTARI_AI_TOKEN, logo, branding (+ User-Agent fix)

[khaledosman]

Brings the Rust SDK in line with the modernized TypeScript SDK (otari-sdk-ts#8 + #9), so the SDKs behave and read consistently.

Behavioral changes

• Hosted-gateway default: in platform mode, the base URL now defaults to https://api.otari.ai when none is configured — so setting OTARI_AI_TOKEN is enough to use the hosted gateway with no api_base. Non-platform mode still requires an explicit base URL.
• Env-var alignment (back-compatible): read canonical OTARI_AI_TOKEN / GATEWAY_API_BASE / GATEWAY_API_KEY first, with the prior OTARI_PLATFORM_TOKEN / OTARI_API_BASE / OTARI_API_KEY kept as legacy aliases (canonical wins). No public function signatures changed; added an additive Otari::api_base() getter.

Docs

• Logo: add the otari seal + wordmark (assets/otari-logo.svg) to the README header.
• Hosted-first README: Quickstart now leads with the hosted path (export OTARI_AI_TOKEN=... + CompletionOptions::default()); added an Authentication section with an env-var table (canonical + legacy aliases).
• Branding cleanup: replace stale any-llm references (logo, nav links, gateway repo link, Docker image ghcr.io/mozilla-ai/otari/gateway, docs links) with their otari equivalents. (The "Based on the proven any-llm Python library" credit is kept — accurate provenance.)
• Provider keys: note that the gateway needs a provider key configured to route upstream.

Bug found & fixed during end-to-end verification

Verified by building a sample app and completing against the live hosted gateway. This surfaced a real bug: the reqwest client sent no User-Agent, and the gateway edge rejects UA-less requests with HTTP 403 — so hosted calls failed while the TS/Python/Go SDKs (which send a default UA) succeeded. Fixed by setting a default otari-rust/<version> User-Agent. Added a regression test asserting the header is sent. The sample now returns a real completion against the hosted gateway.

Implementation note (error detail)

No error-handling change was needed — extract_error_message() already parses the gateway's {"detail": "..."} body, so typed OtariErrors carry the gateway detail. The TS fetch-wrapper fix was specific to the JS OpenAI SDK's "(no body)" quirk.

Verification

• cargo build --all-features → clean
• cargo test --all-features → all pass (incl. new env / hosted-default / User-Agent tests)
• cargo fmt --check + cargo clippy --all-features -- -D warnings → clean
• live hosted completion via a sample app → returns a real response

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