[Task][AI Proxy] Harden configurable retry and fallback policy in AiProxyEngine #7667

Background

Follow up on #7532 to make retry and fallback decisions explicit, configurable, and independently testable. Retry and fallback remain orchestration responsibilities of AiProxyEngine; Transport only reports classified failures.

Scope

• Extract HTTP status and transport failure decisions into AiProxyRetryPolicy.
• Define explicit retry and fallback eligibility for upstream HTTP statuses.
• Define explicit retry and fallback eligibility for classified transport failures.
• Configure retry limits independently for non-streaming and streaming requests.
• Support configurable exponential backoff for non-streaming retries.
• Retry the primary Provider before attempting fallback, and attempt fallback at most once.
• Rebuild fallback requests using the fallback Provider configuration.
• Prevent retry or fallback after response data has been emitted.
• Add unit tests for decision matrices, configuration binding, backoff, fallback, and streaming boundaries.

Out of Scope

• Connection pool, timeout, TLS verification, HTTP proxy, and transport lifecycle configuration.
• Health-based multi-provider load balancing or automatic Provider selection.
• Mapping terminal transport failures to gateway HTTP 502/503/504 responses.
• Protocol and Provider request/response adaptation.

Acceptance Criteria

• Retry and fallback decisions are centralized and independently testable.
• Only explicitly eligible statuses and transport failures are retried or switched to fallback.
• Retry counts and initial backoff can be configured through Spring Boot configuration.
• Streaming retries happen only before the first response chunk is emitted.
• No retry or fallback occurs after downstream response emission starts.
• The fallback Provider is attempted at most once.

Related Issues

• Parent: [Task] Refactor AI Proxy into protocol, provider, and transport layers #7351
• Follow-up to: [Task][AI Proxy] Complete orchestration in AiProxyEngine #7532
• Uses transport failure classification introduced under: [Task][AI Proxy] Harden the reactive AI Transport for production #7384