feat: track step runtime_ms on chat messages (#23219)

## Summary

Adds a `runtime_ms` column to `chat_messages` that records the
wall-clock duration (in milliseconds) of each LLM step. This covers LLM
streaming, tool execution, and retries — the full time the agent is
"alive" for a step.

This is the foundation for billing by agent alive time. The column
follows the same pattern as `total_cost_micros`: stored per assistant
message, aggregatable with `SUM()` over time periods by user.

## Changes

- **Migration**: adds nullable `runtime_ms bigint` to `chat_messages`.
- **chatloop**: adds `Runtime time.Duration` field to `PersistedStep`,
measures `time.Since(stepStart)` at the beginning of each step (covering
stream + tool execution + retries).
- **chatd**: passes `step.Runtime.Milliseconds()` to the assistant
message `InsertChatMessage` call; all other message types (system, user,
tool) get `NULL`.
- **Tests**: adds `runtime > 0` assertion in chatloop tests.

## Billing query pattern

Once ready, aggregation mirrors the existing cost queries:

```sql
SELECT COALESCE(SUM(cm.runtime_ms), 0)::bigint AS total_runtime_ms
FROM chat_messages cm
JOIN chats c ON c.id = cm.chat_id
WHERE c.owner_id = @user_id
  AND cm.created_at >= @start_time
  AND cm.created_at < @end_time
  AND cm.runtime_ms IS NOT NULL;
```

coderd/chatd/chatloop/chatloop.go

@@ -42,6 +42,11 @@ type PersistedStep struct {
 	Content      []fantasy.Content
 	Usage        fantasy.Usage
 	ContextLimit sql.NullInt64
+	// Runtime is the wall-clock duration of this step,
+	// covering LLM streaming, tool execution, and retries.
+	// Zero indicates the duration was not measured (e.g.
+	// interrupted steps).
+	Runtime time.Duration
 }
 
 // RunOptions configures a single streaming chat loop run.
@@ -260,6 +265,7 @@ func Run(ctx context.Context, opts RunOptions) error {
 
 		for step := 0; totalSteps < opts.MaxSteps; step++ {
 			totalSteps++
+			stepStart := time.Now()
 			// Copy messages so that provider-specific caching
 			// mutations don't leak back to the caller's slice.
 			// copy copies Message structs by value, so field
@@ -365,6 +371,7 @@ func Run(ctx context.Context, opts RunOptions) error {
 				Content:      result.content,
 				Usage:        result.usage,
 				ContextLimit: contextLimit,
+				Runtime:      time.Since(stepStart),
 			}); err != nil {
 				if errors.Is(err, ErrInterrupted) {
 					persistInterruptedStep(ctx, opts, &result)

coderd/chatd/chatloop/chatloop_test.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"time"
 
 	"charm.land/fantasy"
 	fantasyanthropic "charm.land/fantasy/providers/anthropic"
@@ -64,6 +65,8 @@ func TestRun_ActiveToolsPrepareBehavior(t *testing.T) {
 	require.Equal(t, 1, persistStepCalls)
 	require.True(t, persistedStep.ContextLimit.Valid)
 	require.Equal(t, int64(4096), persistedStep.ContextLimit.Int64)
+	require.Greater(t, persistedStep.Runtime, time.Duration(0),
+		"step runtime should be positive")
 
 	require.NotEmpty(t, capturedCall.Prompt)
 	require.False(t, containsPromptSentinel(capturedCall.Prompt))