test: add unit tests for lowest-coverage packages

[devin-ai-integration]

Summary

go test -cover ./... surfaced four packages with 0% coverage (no test files at all). This PR adds table-driven unit tests for the three lowest-coverage/untested logic packages plus the pure modelref parser, without touching any production code.

Package	Before	After
internal/modelref	0.0%	100.0%
internal/service/server/trace	0.0%	100.0%
internal/service/server/usage	0.0%	100.0%
internal/service/server/session	0.0%	94.9%

What the new tests cover:

• modelref.Parse — both provider/model and model(provider) forms, whitespace trimming, precedence of the paren form, and fallbacks (empty provider/model inside parens, (x) at index 0, missing ), multi-slash, empty string).
• session.InMemoryManager — GetOrCreate create-vs-reuse, TTL pruning (both explicit Prune and prune-on-lookup), LRU eviction under MaxSessions including recency update on reuse, List snapshot, NewEphemeral not being tracked, and the plugin-registry ExtensionData init/backfill branches. Remaining uncovered lines are the hourly startPruning ticker.
• trace.FileWriter — WriteTrace category dispatch (Chat / Response / Anthropic, individually and combined) verified by inspecting files written to a temp dir, no-category / disabled-tracer / nil-tracer no-ops, and the WriteCategory error-reporting branch (forced via an unwritable root).
• usage.StatsTracker — RecordBilling accounting + cost, CostForRequest computation (no side effects), unknown-model and nil-stats guards.

Tests use dependency injection via small fakes (fakeConfig implementing ConfigAccessor) and real collaborators (plugin.NewRegistry(nil), stats.NewSessionStats, mbtrace.New(...) with t.TempDir()), mirroring the existing test style in the repo.

go test ./... and go vet pass; gofmt clean.

Link to Devin session: https://app.devin.ai/sessions/2db8a2c166aa495e984b9fbcb97fe9bf
Requested by: @ZhiYi-R

[devin-ai-integration]

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