Provider request: Kyma API (OpenAI-compatible, declarative)

## Summary

Request to add Kyma API (https://kymaapi.com) as a declarative provider. Kyma is an OpenAI-compatible endpoint (`https://api.kymaapi.com/v1`, key in `KYMA_API_KEY`) in front of open and frontier language models, with per-model uptime measured from real traffic (https://kymaapi.com/status) and public usage rankings (https://kymaapi.com/rankings).

## Proposed change

A declarative provider in the same shape as the existing aggregator entries: one JSON definition under `crates/goose-providers/src/declarative/definitions/`, one registration line, one row in the providers table of the docs. `dynamic_models: true`, so goose reads the live catalog from `/v1/models` at configure time, with a static fallback list for first run.

The patch is ready on a fork (`kyma-api/goose`, branch `feat/kyma-provider`): `cargo check`, `cargo fmt --check`, `cargo clippy --all-targets -- -D warnings` and `cargo test --lib declarative` (11/11) pass on the pinned toolchain. Opening this issue first per CONTRIBUTING so it can be triaged to Ready before the PR.

## Maintenance

Kyma keeps the definition current when its catalog changes; contact hello@kymaapi.com.

The work is adding a new declarative provider. Start by examining the existing provider definitions in `crates/goose-providers/src/declarative/definitions/` to understand the JSON structure. The patch is ready on a fork, so review the branch `feat/kyma-provider` to see the exact changes. Run the mentioned commands (`cargo check`, `cargo fmt --check`, `cargo clippy`, `cargo test --lib declarative`) to verify the implementation. Done looks like a new JSON file, a registration line, and an updated documentation table.

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