feat(config): add Xiaomi MiMo token plan mode

[Hmbown]

Harvests the narrow Xiaomi MiMo Token Plan mode/env-selection/docs slice from #2627 by @xyuai for the v0.9.0 stewardship branch.

Credits #2621 reported by @springeye. This does not close #2621; remaining scope there includes model-list behavior, credit/cost display, and rate-limit awareness.

What landed:

• Adds providers.xiaomi_mimo.mode plus XIAOMI_MIMO_MODE / MIMO_MODE.
• Supports token-plan-sgp, token-plan-cn, token-plan-ams, and pay-as-you-go endpoint selection.
• Prefers XIAOMI_MIMO_TOKEN_PLAN_API_KEY / MIMO_TOKEN_PLAN_API_KEY for Token Plan mode while preserving standard MiMo key env vars for pay-as-you-go.
• Keeps the v0.9 branch's existing Token Plan api-key header behavior.
• Updates README/config/provider docs and changelog credit.

Verification:

• cargo test -p codewhale-config --locked xiaomi -- --nocapture
• cargo test -p codewhale-tui --locked --bin codewhale-tui xiaomi_mimo -- --nocapture
• cargo fmt --all -- --check
• cargo clippy -p codewhale-config -p codewhale-tui --locked --all-targets --all-features -- -D warnings
• git diff --check
• git diff --cached --check
• ./scripts/release/check-versions.sh
• python3 scripts/check-coauthor-trailers.py --range origin/codex/v0.9.0-stewardship..HEAD --check-authors

[greptile-apps]

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[gemini-code-assist]

Code Review

This pull request introduces Xiaomi MiMo mode selection (XIAOMI_MIMO_MODE / MIMO_MODE) to support Token Plan region endpoints (China, Singapore, Amsterdam) and pay-as-you-go routing, along with dedicated Token Plan environment keys. Feedback on the implementation points out that when both mode and base_url are provided, base_url inference can incorrectly override an explicitly configured mode during API key resolution. The reviewer suggests prioritizing explicit mode configuration over base_url inference in both crates/config/src/lib.rs and crates/tui/src/config.rs.

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[gemini-code-assist]

In the current implementation, if both mode and base_url are provided, the base_url inference can override an explicitly configured mode. For example, if mode is set to "token-plan-cn" but base_url is configured to a pay-as-you-go endpoint, standard_selected will evaluate to true, selecting the standard API key instead of the token plan API key. Explicit configuration of mode should take precedence over base_url inference. We can resolve this by matching on normalized_mode first, and only falling back to base_url inference if normalized_mode is None.

    let (standard_selected, token_plan_selected) = match normalized_mode.as_deref() {
        Some(m) => (
            xiaomi_mimo_mode_uses_standard_endpoint(m),
            xiaomi_mimo_base_url_for_mode(m).is_some(),
        ),
        None => (
            base_url.is_some_and(xiaomi_mimo_base_url_is_pay_as_you_go),
            base_url.is_some_and(xiaomi_mimo_base_url_uses_token_plan),
        ),
    };
    if standard_selected {
        return xiaomi_mimo_env_var(STANDARD_ENV_VARS);
    }
    if token_plan_selected {
        return xiaomi_mimo_env_var(TOKEN_PLAN_ENV_VARS);
    }

[gemini-code-assist]

In the current implementation, if both mode and base_url are provided, the base_url inference can override an explicitly configured mode. For example, if mode is set to "token-plan-cn" but base_url is configured to a pay-as-you-go endpoint, standard_selected will evaluate to true, selecting the standard API key instead of the token plan API key. Explicit configuration of mode should take precedence over base_url inference. We can resolve this by matching on normalized_mode first, and only falling back to base_url inference if normalized_mode is None.

    let (standard_selected, token_plan_selected) = match normalized_mode.as_deref() {
        Some(m) => (
            xiaomi_mimo_mode_uses_standard_endpoint(m),
            xiaomi_mimo_base_url_for_mode(m).is_some(),
        ),
        None => (
            base_url.is_some_and(xiaomi_mimo_base_url_is_pay_as_you_go),
            base_url.is_some_and(xiaomi_mimo_base_url_uses_token_plan),
        ),
    };
    if standard_selected {
        return xiaomi_mimo_env_var(STANDARD_ENV_VARS);
    }
    if token_plan_selected {
        return xiaomi_mimo_env_var(TOKEN_PLAN_ENV_VARS);
    }