v1.12.4-rc1: extract budget, sentinels, xml-parser from inference.ts

Pure file moves. No behavior change. inference.ts retains createInferenceRunner
public surface; new files are internal to services/inference/.

- budget.ts: resolveToolBudget
- sentinels.ts: detectDoomLoop (re-exported through inference.ts),
  isCapHitSentinel, isDoomLoopSentinel, isAnySentinel
- xml-parser.ts: parseXmlToolCall, partialXmlOpenerStart

First of four refactor batches preparing inference.ts for the v1.13
AI SDK migration. inference.ts goes from 1780 LoC to ~1620.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/server/src/services/inference/budget.ts

@@ -0,0 +1,20 @@
+import type { Agent } from '../../types/api.js';
+import { READ_ONLY_TOOL_NAMES } from '../tools.js';
+
+// v1.8.2: tool-call budget defaults. Resolved per-turn by resolveToolBudget.
+//   - Agent with explicit max_tool_calls: that value.
+//   - Agent with read-only-only tools:    BUDGET_READ_ONLY (30).
+//   - Agent with any non-read-only tool:  BUDGET_NON_READ_ONLY (10).
+//   - No agent (raw chat):                BUDGET_NO_AGENT (15).
+export const BUDGET_READ_ONLY = 30;
+export const BUDGET_NON_READ_ONLY = 10;
+export const BUDGET_NO_AGENT = 15;
+
+const READ_ONLY_SET: ReadonlySet<string> = new Set(READ_ONLY_TOOL_NAMES);
+
+export function resolveToolBudget(agent: Agent | null): number {
+  if (agent?.max_tool_calls != null) return agent.max_tool_calls;
+  if (!agent) return BUDGET_NO_AGENT;
+  const allReadOnly = agent.tools.every((t) => READ_ONLY_SET.has(t));
+  return allReadOnly ? BUDGET_READ_ONLY : BUDGET_NON_READ_ONLY;
+}