refactor(telemetry): reduce PrintTraceSummary positional params and add output assertions

[lizhengfeng101]

Description

PrintTraceSummary in internal/telemetry/events.go currently takes 9 positional parameters (after #870 merged the sessionID param). This makes calls hard to read and error-prone as the signature grows. Additionally, all existing tests for this function are smoke tests that only verify "no panic" — they cannot assert the actual printed output because stdout.Writer() captures os.Stdout at init time and there is no swap/redirect mechanism for tests.

This issue covers two related improvements:

1. Refactor the function signature

Current signature (after #870):

func PrintTraceSummary(filesReviewed, commentsGenerated int64,
    inputTokens, outputTokens, totalTokens int64,
    cacheReadTokens, cacheWriteTokens int64,
    duration time.Duration, sessionID string)

Option A (preferred): Introduce a TraceSummary struct and accept it as a single parameter:

type TraceSummary struct {
    FilesReviewed    int64
    CommentsGenerated int64
    InputTokens      int64
    OutputTokens     int64
    TotalTokens      int64
    CacheReadTokens  int64
    CacheWriteTokens int64
    Duration         time.Duration
    SessionID        string
}

func PrintTraceSummary(s TraceSummary) { ... }

Option B: Extract session printing into a separate PrintSessionID(id string) helper and keep the remaining 8-param signature (less ideal long-term but simpler diff).

2. Add a stdout.Swap test helper for output assertions

The internal/stdout package currently exposes Writer() and Quiet(). A new Swap(w io.Writer) func() (or a test-only CaptureOutput(t *testing.T) *bytes.Buffer) would allow tests to redirect output and assert content:

// Swap replaces the stdout writer and returns a restore function.
func Swap(replacement io.Writer) func() {
    mu.Lock()
    old := w
    w = replacement
    mu.Unlock()
    return func() {
        mu.Lock()
        w = old
        mu.Unlock()
    }
}

Then tests can assert actual output:

func TestPrintTraceSummary_WithSessionID(t *testing.T) {
    var buf bytes.Buffer
    defer stdout.Swap(&buf)()
    PrintTraceSummary(TraceSummary{..., SessionID: "abc-123"})
    assert.Contains(t, buf.String(), "[ocr] Session: abc-123")
}

Scope

• File(s): internal/telemetry/events.go, internal/telemetry/events_test.go, internal/stdout/stdout.go
• Function/area: PrintTraceSummary, stdout package
• Call site: cmd/opencodereview/shared.go (update to use struct)

Acceptance Criteria

• PrintTraceSummary accepts a struct (or the session line is extracted to a helper)
• internal/stdout exposes a Swap (or test-capture) mechanism
• PrintTraceSummary tests assert actual output strings, not just "no panic"
• All existing tests updated for the new signature
• Tests pass (make test)
• Code check passes (make check)
• Coverage maintained ≥90% (make coverage)

Context

Follow-up from #870 (which closed #866). The PR author flagged both items as out-of-scope improvements. See the PR description "Design notes" section for rationale.