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/database/db2sdk/db2sdk_test.go

@@ -467,7 +467,7 @@ func TestChatMessage_PreservesProviderExecutedOnToolResults(t *testing.T) {
 	dbMsg := database.ChatMessage{
 		ID:     1,
 		ChatID: uuid.New(),
-		Role:   string(fantasy.MessageRoleAssistant),
+		Role:   string(codersdk.ChatMessageRoleAssistant),
 		Content: pqtype.NullRawMessage{
 			RawMessage: rawContent,
 			Valid:      true,