refactor(chat): give provider message conversion its own module

CONTEXT.md

@@ -90,3 +90,12 @@ live, durable, diagnostic, and usage Adapters (Phase 5, separately designed).
 Through Gate 4, hosted durable and AG-UI production projections still consume
 compatibility UI chunks, and production runs stay on stream protocol
 version 1.
+
+## Provider Message Conversion
+
+The single owner of turning a chat's replay history into the ordered message
+list a provider sees: `src/chat/provider-message-conversion.ts`. It asks Tool
+Replay Reconciliation which tool occurrences are authoritative, maps each role's
+parts into provider content, and settles into one `ProviderModelMessage[]`.
+Message preparation and the compatibility layer are callers; neither re-derives
+the mapping.

scripts/lint/ban-chat-antipatterns.ts

@@ -113,11 +113,13 @@ const FILE_SIZE_CEILINGS: Record<string, number> = {
   // real seams (part-field-access, message-part-parsing, tool-replay
   // reconciliation). Not React components, so this map does not subject them
   // to the antipattern ratchets above — it only pins their size.
-  "src/chat/conversation.ts": 1018,
+  "src/chat/conversation.ts": 557,
   "src/chat/message-prep.ts": 2016,
-  "src/chat/tool-replay-reconciliation.ts": 294,
+  "src/chat/tool-replay-reconciliation.ts": 291,
   "src/chat/message-part-parsing.ts": 264,
-  "src/chat/part-field-access.ts": 66,
+  "src/chat/part-field-access.ts": 65,
+  "src/chat/provider-input-types.ts": 34,
+  "src/chat/provider-message-conversion.ts": 447,
 };
 
 function checkFileSizes(): boolean {

src/agent/hosted/chat-request.test.ts

@@ -1,5 +1,5 @@
 import "#veryfront/schemas/_test-setup.ts";
-import { convertUiMessagesToProviderModelMessages } from "#veryfront/chat/conversation";
+import { convertUiMessagesToProviderModelMessages } from "../../chat/provider-message-conversion.ts";
 import { assertEquals, assertStringIncludes } from "#veryfront/testing/assert.ts";
 import { describe, it } from "#veryfront/testing/bdd.ts";
 import { DEFAULT_MAX_BODY_SIZE_BYTES } from "#veryfront/utils/constants/index.ts";

src/chat/compat.ts

@@ -26,13 +26,13 @@ import {
   getAgUiWireEventSchema,
 } from "./ag-ui.ts";
 import {
-  convertUiMessagesToProviderModelMessages,
   getApiConversationSchema,
   getApiMessageSchema,
   getConversationTypeSchema,
   getMessagePartSchema,
   getMessageStatusSchema,
 } from "./conversation.ts";
+import { convertUiMessagesToProviderModelMessages } from "./provider-message-conversion.ts";
 import {
   prepareProviderModelMessagesFromUiMessages,
   sanitizeProviderModelMessages,

src/chat/conversation.test.ts

@@ -5,7 +5,7 @@ import type {
   ChatProviderModelInputMessage,
   ChatProviderModelInputPart,
   ChatProviderModelInputToolResultPart,
-} from "#veryfront/chat/conversation";
+} from "./provider-input-types.ts";
 import type {
   ChatToolCallPart,
   ChatToolPartState,
@@ -21,7 +21,6 @@ import {
   messageStatusSchema,
 } from "#veryfront/chat/compat";
 import {
-  convertUiMessagesToProviderModelMessages,
   extractTextFromMessage,
   extractUploadId,
   hasIncompleteToolParts,
@@ -34,6 +33,7 @@ import {
   stringifyUnknown,
   toConversationPartsFromUiMessage,
 } from "#veryfront/chat/conversation";
+import { convertUiMessagesToProviderModelMessages } from "./provider-message-conversion.ts";
 
 const GITHUB_PR_DIFF_INPUT = { owner: "veryfront", repo: "veryfront-code", pull_number: 3092 };
 const GITHUB_LIST_PRS_INPUT = { owner: "veryfront", repo: "veryfront-code" };