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/chatloop/compaction.go

@@ -55,7 +55,7 @@ type CompactionOptions struct {
 	// PublishMessagePart publishes streaming parts to connected
 	// clients so they see "Summarizing..." / "Summarized" UI
 	// transitions during compaction.
-	PublishMessagePart func(fantasy.MessageRole, codersdk.ChatMessagePart)
+	PublishMessagePart func(codersdk.ChatMessageRole, codersdk.ChatMessagePart)
 
 	OnError func(error)
 }
@@ -110,12 +110,8 @@ func tryCompact(
 	// connected clients see activity during summary generation.
 	if config.PublishMessagePart != nil && config.ToolCallID != "" {
 		config.PublishMessagePart(
-			fantasy.MessageRoleAssistant,
-			codersdk.ChatMessagePart{
-				Type:       codersdk.ChatMessagePartTypeToolCall,
-				ToolCallID: config.ToolCallID,
-				ToolName:   config.ToolName,
-			},
+			codersdk.ChatMessageRoleAssistant,
+			codersdk.ChatMessageToolCall(config.ToolCallID, config.ToolName, nil),
 		)
 	}
 
@@ -163,13 +159,8 @@ func tryCompact(
 			"context_limit_tokens": contextLimit,
 		})
 		config.PublishMessagePart(
-			fantasy.MessageRoleTool,
-			codersdk.ChatMessagePart{
-				Type:       codersdk.ChatMessagePartTypeToolResult,
-				ToolCallID: config.ToolCallID,
-				ToolName:   config.ToolName,
-				Result:     resultJSON,
-			},
+			codersdk.ChatMessageRoleTool,
+			codersdk.ChatMessageToolResult(config.ToolCallID, config.ToolName, resultJSON, false),
 		)
 	}
 
@@ -186,14 +177,8 @@ func publishCompactionError(config CompactionOptions, msg string) {
 		"error": msg,
 	})
 	config.PublishMessagePart(
-		fantasy.MessageRoleTool,
-		codersdk.ChatMessagePart{
-			Type:       codersdk.ChatMessagePartTypeToolResult,
-			ToolCallID: config.ToolCallID,
-			ToolName:   config.ToolName,
-			Result:     errJSON,
-			IsError:    true,
-		},
+		codersdk.ChatMessageRoleTool,
+		codersdk.ChatMessageToolResult(config.ToolCallID, config.ToolName, errJSON, true),
 	)
 }