refactor(llm): redesign error model as flat tagged union

- Replace LLMError { module, method, reason } wrapper with a flat tagged
  union: BadRequest, Authentication, PermissionDenied, NotFound, RateLimit,
  QuotaExceeded, ContentPolicy, ContextOverflow, ServerError, APIError,
  ConnectionError, TimeoutError, MalformedResponse, NoRoute.
- Delete the provider-error LLMEvent: streams carry output only and every
  failure exits through the typed error channel.
- Add one shared classifyApiFailure classifier used by the HTTP executor,
  protocol stream errors, and the AI SDK adapter so all routes classify
  identically (including OpenAI in-stream rate_limit_exceeded and
  internal_error codes).
- Enforce a terminal contract in LLMClient.stream for every route: EOF
  without finish and output after finish fail as MalformedResponse.
- Classify AI SDK failures properly in core/aisdk.ts instead of collapsing
  to UnknownProvider; preserve status, headers, body, and retry-after.
- Simplify the session runner: drop held-back overflow events, key overflow
  recovery off LLM.ContextOverflow, retry RateLimit | ServerError |
  ConnectionError | TimeoutError.
- Map new tags in toSessionError (provider.context-overflow,
  provider.timeout, provider.not-found).

packages/core/test/session-execution.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, test } from "bun:test"
-import { LLMError, TransportReason } from "@opencode-ai/llm"
+import { ConnectionError } from "@opencode-ai/llm"
 import { Database } from "@opencode-ai/core/database/database"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/core/effect/layer-node"
@@ -25,17 +25,10 @@ const it = testEffect(AppNodeBuilder.build(LayerNode.group([Database.node, Event
 describe("SessionExecution lifecycle", () => {
   test("classifies success and typed failure terminals", () => {
     expect(SessionExecution.terminal(Exit.succeed(undefined))).toEqual({ type: "succeeded" })
-    expect(
-      SessionExecution.terminal(
-        Exit.fail(
-          new LLMError({
-            module: "test",
-            method: "stream",
-            reason: new TransportReason({ message: "Disconnected" }),
-          }),
-        ),
-      ),
-    ).toEqual({ type: "failed", error: { type: "provider.transport", message: "Disconnected" } })
+    expect(SessionExecution.terminal(Exit.fail(new ConnectionError({ message: "Disconnected" })))).toEqual({
+      type: "failed",
+      error: { type: "provider.transport", message: "Disconnected" },
+    })
     const storage = new ToolOutputStore.StorageError({ operation: "encode", cause: new Error("invalid output") })
     expect(SessionExecution.terminal(Exit.fail(storage))).toEqual({
       type: "failed",