feat(whaleflow): add model role policy registry

[Hmbown]

Summary

• Adds ModelRole, ModelCapabilities, and a crate-local ProviderRegistry for role/capability-based WhaleFlow model selection.
• Adds ModelProvider, CompletionRequest, CompletionResponse, and a MockModelProvider foundation without live provider calls.
• Adds fail-closed parse_json_with_repair / repair_json_text_once for the WhaleFlow: model-provider registry and role-based model policy #2672 JSON fallback shape.
• Covers role defaults, provider capability fallback, agent-role mapping, mock provider response, and JSON repair/fail-closed tests.

Stewardship Notes

• Advances WhaleFlow: model-provider registry and role-based model policy #2672 as a narrow model-agnostic foundation.
• This intentionally does not wire runtime provider routing, OpenAI-compatible calls, Arcee smoke tests, billing-affecting fallback, or TUI provider switching.
• Keeps the broader WhaleFlow direction credited to @AdityaVG13 while leaving feat: add WhaleFlow — declarative multi-agent workflow orchestration #2482/Feat/whaleflow cost tracking #2486 open as wider draft branches.

Verification

• cargo test -p codewhale-whaleflow --locked
• cargo clippy -p codewhale-whaleflow --all-targets --locked -- -D warnings
• cargo fmt --all -- --check
• ./scripts/release/check-versions.sh
• ./scripts/release/check-ohos-deps.sh
• git diff --check

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