Fix channel monitor false errors for Claude and Gemini

[Go1c]

Summary

• Fix channel monitor false errors for Claude/Gemini probes.
• Preserve selected monitor request-template headers for Claude Code probes instead of overwriting them, and normalize duplicate header names case-insensitively so template/user values win deterministically.
• Keep default monitor probes unchanged for normal upstreams, but when Anthropic explicitly rejects a probe as non-Claude-Code usage, automatically retry with the same Claude Code-style request shape used by real Claude Code traffic.
• Regenerate Claude Code monitor fingerprints on that rejection path: metadata.user_id and x-client-request-id are fresh on retry.
• Improve Claude/Gemini monitor response extraction for thinking/SSE output.
• Retry once on transient upstream 5xx/Cloudflare responses and response-header timeouts.

Fixes #1946

Test Plan

• From backend/: GOCACHE=/tmp/sub2api-go-build-cache GOMODCACHE=/tmp/sub2api-go-mod-cache go test -count=1 -tags=unit ./internal/service -run "RunCheckForModel_AnthropicCompatibilityProbePreservesClaudeCodeTemplateHeaders|TestMergeHeaders_NormalizesDuplicateHeaderNames" -v
• From backend/: GOCACHE=/tmp/sub2api-go-build-cache GOMODCACHE=/tmp/sub2api-go-mod-cache go test -count=1 -tags=unit ./internal/service -run "ChannelMonitor|RunCheckForModel|Challenge"
• From backend/: GOCACHE=/tmp/sub2api-go-build-cache GOMODCACHE=/tmp/sub2api-go-mod-cache go test ./...
• From frontend/: npm run build

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[Go1c]

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[Go1c]

I have read the CLA Document and I hereby sign the CLA