refactor(llm): replace LLMError reasons with flat tagged union

Replace the LLMError { module, method, reason } wrapper with a flat
tagged union (LLM.BadRequest, LLM.Authentication, LLM.PermissionDenied,
LLM.NotFound, LLM.RateLimit, LLM.QuotaExceeded, LLM.ContentPolicy,
LLM.ContextOverflow, LLM.ServerError, LLM.APIError, LLM.ConnectionError,
LLM.TimeoutError, LLM.MalformedResponse, LLM.NoRoute) plus an isLLMError
guard. Add one shared classifyApiFailure classifier used by the HTTP
executor and the AI SDK adapter so both surfaces classify identically,
preserving status, headers, body, and retry-after.

Core policy moves onto tags: retry RateLimit | ServerError |
ConnectionError | TimeoutError; toSessionError adds
provider.context-overflow, provider.timeout, and provider.not-found.

The provider-error stream event and the runner's held-back overflow
handling are unchanged here; isContextOverflowFailure now bridges old
events and new tags until the event is removed.

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

@@ -1,15 +1,16 @@
 import { describe, expect, test } from "bun:test"
 import {
+  BadRequest,
+  ConnectionError,
+  ContextOverflow,
   LLMClient,
-  LLMError,
   LLMEvent,
+  MalformedResponse,
   Model,
+  RateLimit,
   ToolFailure,
-  TransportReason,
-  InvalidProviderOutputReason,
-  InvalidRequestReason,
-  RateLimitReason,
   type LLMClientShape,
+  type LLMError,
   type LLMRequest,
 } from "@opencode-ai/llm"
 import * as OpenAIChat from "@opencode-ai/llm/protocols/openai-chat"
@@ -483,26 +484,11 @@ const setup = Effect.gen(function* () {
   return yield* SessionV2.Service
 })
 
-const providerUnavailable = () =>
-  new LLMError({
-    module: "test",
-    method: "stream",
-    reason: new TransportReason({ message: "Provider unavailable" }),
-  })
+const providerUnavailable = () => new ConnectionError({ message: "Provider unavailable" })
 
-const invalidRequest = () =>
-  new LLMError({
-    module: "test",
-    method: "stream",
-    reason: new InvalidRequestReason({ message: "Invalid request" }),
-  })
+const invalidRequest = () => new BadRequest({ message: "Invalid request" })
 
-const rateLimited = (retryAfterMs?: number) =>
-  new LLMError({
-    module: "test",
-    method: "stream",
-    reason: new RateLimitReason({ message: "Rate limited", retryAfterMs }),
-  })
+const rateLimited = (retryAfterMs?: number) => new RateLimit({ message: "Rate limited", retryAfterMs })
 
 const setupOverflowRecovery = Effect.gen(function* () {
   const session = yield* setup
@@ -1985,16 +1971,7 @@ describe("SessionRunnerLLM", () => {
   it.effect("recovers once from a raw context overflow failure", () =>
     Effect.gen(function* () {
       const session = yield* setupOverflowRecovery
-      responseStream = Stream.fail(
-        new LLMError({
-          module: "test",
-          method: "stream",
-          reason: new InvalidRequestReason({
-            message: "prompt too long",
-            classification: "context-overflow",
-          }),
-        }),
-      )
+      responseStream = Stream.fail(new ContextOverflow({ message: "prompt too long" }))
       responses = [
         reply.text("## Objective\n- Recover raw overflow", "text-summary"),
         reply.text("Recovered", "text-final"),
@@ -3892,11 +3869,7 @@ describe("SessionRunnerLLM", () => {
     Effect.gen(function* () {
       const session = yield* setup
       yield* admit(session, "Call a malformed tool")
-      const failure = new LLMError({
-        module: "test",
-        method: "stream",
-        reason: new InvalidProviderOutputReason({ message: "Invalid JSON input for tool call echo" }),
-      })
+      const failure = new MalformedResponse({ message: "Invalid JSON input for tool call echo" })
       responseStream = Stream.fromIterable([
         LLMEvent.stepStart({ index: 0 }),
         LLMEvent.toolInputStart({ id: "call-malformed", name: "echo" }),