feat(coderd/chatd): unify chat storage on SDK parts and fix file-reference rendering (#22958)

File-reference parts in user messages were flattened to `TextContent` at
write time because fantasy has no file-reference content type. The
frontend never saw them as structured parts.

This moves all write paths (user, assistant, tool) from fantasy envelope
format to `codersdk.ChatMessagePart`. The streaming layer (`chatloop`)
is untouched, the conversion happens at the serialization boundary in
`persistStep`.

Old rows are still readable. `ParseContent` uses a structural heuristic
(`isFantasyEnvelopeFormat`) to distinguish legacy envelopes from SDK
parts. We chose this over try/fallback because fantasy envelopes
partially unmarshal into `ChatMessagePart` (the `type` field matches)
while silently losing content. A guard test enforces that no SDK part
can produce the envelope shape.

This is forward-only: new rows are unreadable by old code. Chat is
behind a feature flag so rollback risk is contained.

Also adds a typed `ChatMessageRole` to replace raw strings and
`fantasy.MessageRole*` casts at the persistence boundary. The type
covers `ChatMessage.Role`, `ChatStreamMessagePart.Role`, the
`PublishMessagePart` callback chain, and all DB write sites.
`fantasy.MessageRole*` remains only where we build `fantasy.Message`
structs for LLM dispatch.

Separately, `ProviderMetadata` was leaking to SSE clients via
`publishMessagePart`. `StripInternal` now runs on both the SSE and REST
paths, covering this.

Other cleanup:

- Old `db2sdk.contentBlockToPart` silently dropped metadata on
text/reasoning/tool-call content. New code preserves it.
- `providerMetadataToOptions` now logs warnings instead of silently
returning nil.
- `db2sdk` shrinks from ~250 lines of parallel conversion to ~15 lines
delegating to `chatprompt.ParseContent()`, removing the `fantasy` import
entirely.

Refs #22821

coderd/chatd/quickgen.go

@@ -21,6 +21,7 @@ import (
 	"github.com/coder/coder/v2/coderd/chatd/chatretry"
 	"github.com/coder/coder/v2/coderd/database"
 	coderdpubsub "github.com/coder/coder/v2/coderd/pubsub"
+	"github.com/coder/coder/v2/codersdk"
 )
 
 const titleGenerationPrompt = "You are a title generator. Your ONLY job is to output a short title (2-8 words) " +
@@ -158,13 +159,13 @@ func titleInput(
 		}
 
 		switch message.Role {
-		case string(fantasy.MessageRoleAssistant), string(fantasy.MessageRoleTool):
+		case string(codersdk.ChatMessageRoleAssistant), string(codersdk.ChatMessageRoleTool):
 			return "", false
-		case string(fantasy.MessageRoleUser):
+		case string(codersdk.ChatMessageRoleUser):
 			userCount++
 			if firstUserText == "" {
 				parsed, err := chatprompt.ParseContent(
-					string(fantasy.MessageRoleUser), message.Content,
+					codersdk.ChatMessageRoleUser, message.Content,
 				)
 				if err != nil {
 					return "", false
@@ -226,22 +227,21 @@ func fallbackChatTitle(message string) string {
 	return truncateRunes(title, maxRunes)
 }
 
-// contentBlocksToText concatenates the text parts of content blocks
-// into a single space-separated string.
-func contentBlocksToText(content []fantasy.Content) string {
-	parts := make([]string, 0, len(content))
-	for _, block := range content {
-		textBlock, ok := fantasy.AsContentType[fantasy.TextContent](block)
-		if !ok {
+// contentBlocksToText concatenates the text parts of SDK chat
+// message parts into a single space-separated string.
+func contentBlocksToText(parts []codersdk.ChatMessagePart) string {
+	texts := make([]string, 0, len(parts))
+	for _, part := range parts {
+		if part.Type != codersdk.ChatMessagePartTypeText {
 			continue
 		}
-		text := strings.TrimSpace(textBlock.Text)
+		text := strings.TrimSpace(part.Text)
 		if text == "" {
 			continue
 		}
-		parts = append(parts, text)
+		texts = append(texts, text)
 	}
-	return strings.Join(parts, " ")
+	return strings.Join(texts, " ")
 }
 
 func truncateRunes(value string, maxLen int) string {
@@ -343,7 +343,13 @@ func generateShortText(
 		return "", xerrors.Errorf("generate short text: %w", err)
 	}
 
-	text := strings.TrimSpace(contentBlocksToText(response.Content))
+	responseParts := make([]codersdk.ChatMessagePart, 0, len(response.Content))
+	for _, block := range response.Content {
+		if p := chatprompt.PartFromContent(block); p.Type != "" {
+			responseParts = append(responseParts, p)
+		}
+	}
+	text := strings.TrimSpace(contentBlocksToText(responseParts))
 	text = strings.Trim(text, "\"'`")
 	return text, nil
 }