feat(ai-proxy): add anonymous affinity cookie fallback

[hongshi1]

Ⅰ. Describe what this PR did

Added anonymous affinity cookie fallback for stateful AI APIs in ai-proxy. When x-mse-consumer is absent, the first successful response returns a higress-ai-affinity cookie and subsequent requests use that cookie for stable token selection.

Ⅱ. Does this pull request fix one issue?

fixes #3473

Ⅲ. Why don't you add test cases (unit test/integration test)?

Added provider unit tests and TestConsumerAffinity regression coverage for anonymous cookie fallback.

Ⅳ. Describe how to verify it

Run go test ./provider/... and go test . -run TestConsumerAffinity -count=1 -timeout 60s under plugins/wasm-go/extensions/ai-proxy.

Ⅴ. Special notes for reviews

This change only affects stateful APIs with multiple apiTokens configured and keeps x-mse-consumer as the first-priority affinity key.

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