coder(providers): v2.3 provider-lifecycle phase 4 — config HTTP API (diagnostic returns JSON)

GET/PATCH /api/providers/config, subset POST /refresh, and
GET /api/providers/:id/diagnostic (JSON { diagnostic }, §6.4). PATCH order
is validate→save→reload→clear; a malformed body or invalid merged config
returns 422 without writing, and a save failure returns 500 without
reloading (no file/registry divergence). Web client + types extended.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/services/__tests__/provider-snapshot.test.ts

@@ -7,6 +7,7 @@ import {
   prefixLlamaSwapModels,
   clearProviderSnapshotCache,
   getProviderSnapshot,
+  peekSnapshotEntry,
 } from '../provider-snapshot.js';
 import { loadProviderConfig } from '../provider-config-registry.js';
 
@@ -324,6 +325,18 @@ describe('getProviderSnapshot', () => {
     expect(claude!.models.find((m) => m.id === 'claude-opus-4-8')?.thinkingOptions?.length).toBeGreaterThan(0);
   });
 
+  it('peekSnapshotEntry returns a cached entry (read-only) and undefined when cold/unknown', async () => {
+    loadConfigFixture({});
+    // Cold cache → undefined (no build triggered).
+    expect(peekSnapshotEntry('boocode', '/tmp/peek')).toBeUndefined();
+
+    const sql = mockSql([]);
+    await getProviderSnapshot(sql, config, '/tmp/peek', true);
+
+    expect(peekSnapshotEntry('boocode', '/tmp/peek')?.name).toBe('boocode');
+    expect(peekSnapshotEntry('does-not-exist', '/tmp/peek')).toBeUndefined();
+  });
+
   it('2.7 warm cache: a second snapshot within the warm window spawns ZERO probes', async () => {
     loadConfigFixture({});
     mockProbe.mockResolvedValue({