Keep token usage label maps within their bound when merged

[azooz2003-bit]

`addTokenUsageLabelCounts` let a label map fill all `tokenUsageMaxLabelKeys` slots with real labels and then added `other` as a 17th key. When a snapshot merged such a map into an empty one, whether the result stayed at 16 depended on map iteration order, so `TestTokenUsageStopReasonAndErrorLabelsStayBounded` failed roughly one run in seventeen (seen on #397: `stop reasons = 17 keys, want at most 16`).

The fix reserves the last slot for `other`, so a map never exceeds the bound regardless of merge order. Adds a test that merges a full map (16 labels plus `other`) repeatedly and asserts the bound and the total.

Test: `go test -count=20 -run 'TestTokenUsage|TestAddTokenUsageLabel' ./internal/proxy/` passes.

🤖 Generated with Claude Code

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Summary by cubic

Fixes addTokenUsageLabelCounts so merged token usage label maps never exceed the tokenUsageMaxLabelKeys bound. Previously, a full map could add other as a 17th key, and whether that key survived merging depended on map iteration order, causing TestTokenUsageStopReasonAndErrorLabelsStayBounded to fail intermittently. The last slot is now reserved for other, and a test repeatedly merges a full map to assert the bound and total.

^{Written for commit cdcadad. Summary will update on new commits.}

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Migrated from #468 after correcting the PR author identity. The original head commit cdcadad is preserved.

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Reviewing files that changed from the base of the PR and between 90c61ae and cdcadad.

📒 Files selected for processing (2)

• internal/proxy/token_usage.go
• internal/proxy/token_usage_metadata_test.go

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