refactor: remove unused tsm rr fallback

[houyuwushang]

Description

Fixes #1009.

TSM was still constructing a round-robin fallback handler, but ServeHTTP no longer uses it. Non-merge paths, missing request resources, and the single-member fast path already dispatch directly to the first live pool target.

This removes the unused rr dependency, adds a test that pins the current non-merge fallback behavior, and documents that fallback in the ALB TSM docs.

Type of Change

• • Bug fix
• • New feature
• • Optimization
• • Test coverage
• • Documentation
• • Infrastructure

AI Disclosure

• • This contribution DOES NOT include AI-generated changes
• • This contribution DOES include AI-generated changes, and I have reviewed the relevant contributing guidelines.

Tests

• go test ./pkg/backends/alb/mech/tsm -run 'Test(New|HandlerStopPool|TSMNonMergePathUsesFirstLiveMember|HandleResponseMerge)$' -count=1
• go test ./pkg/backends/alb/mech/tsm -count=1
• go test ./pkg/backends/alb/... -count=1
• go tool golangci-lint run --timeout 5m ./pkg/backends/alb/mech/tsm
• git diff --check (only local Windows autocrlf warnings)

[jranson]

@houyuwushang in this change it would be good to describe the non-merge fallback behavior (dispatching to the first live pool target) in the ALB TSM Documentation. Otherwise, looks good.

[houyuwushang]

Thanks, added that note to the TSM docs. The local TSM/ALB package tests still pass, and CI is green.