feat: add NEAR AI Cloud provider

[PierreLeGuen]

Summary

Adds NEAR AI Cloud as a built-in OpenAI-compatible inference provider with NEARAI_API_KEY authentication and the default https://cloud-api.near.ai/v1 base URL.

Changes

• Registers nearai in provider defaults, provider detection, and the model catalog.
• Adds five NEAR AI Cloud TEE-backed chat models with pricing, context windows, aliases, and provider metadata.
• Documents NEAR AI Cloud setup in provider/configuration docs and environment examples.

Testing

• cargo fmt --all -- --check
• cargo test -p openfang-runtime nearai
• cargo test -p openfang-runtime test_known_providers_list
• cargo test -p openfang-runtime test_catalog_has_providers
• cargo build -p openfang-runtime --lib
• cargo clippy -p openfang-runtime --all-targets -- -D warnings
• cargo build --workspace --lib --exclude openfang-desktop

Full cargo build --workspace --lib requires the desktop GTK/GLib pkg-config dependencies; the non-desktop workspace library build passed locally.

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries