feat: DSML tool-call recovery plugin for OpenCode V2

Recovers DeepSeek DSML tool calls that leak as assistant text: streaming
middleware (AI-SDK path), SSE rewrite (native path), history sanitization,
preventive directive, conditional system nudge, and a capped idle watchdog.
116 tests, synthetic golden fixtures.

test/fixtures.ts

@@ -0,0 +1,58 @@
+/**
+ * Fixture builders for DSML parser tests.
+ *
+ * The live DSML marker contains U+FF5C (full-width vertical bar). We never write
+ * that byte literally in this repository: models and greedy provider extractors
+ * treat it as a tool-call start, and a literal token in a source file becomes a
+ * phantom call. Everything is built from `BAR` at runtime instead.
+ */
+
+/** U+FF5C full-width vertical bar. */
+export const BAR = String.fromCharCode(0xff5c)
+
+/** ASCII pipe, the other bar the grammar accepts. */
+export const PIPE = "|"
+
+/** Canonical marker: `<｜DSML｜>`. */
+export const marker = (bar = BAR, count = 1) => `${bar.repeat(count)}DSML${bar.repeat(count)}`
+
+/** Mangled / doubled marker, as emitted by gateways that re-decode tokens. */
+export const markerDoubled = () => marker(BAR, 2)
+export const markerAscii = () => "|DSML|"
+
+/** A DSML tag: `<MARKERname attrs...>`. */
+export const tag = (name: string, attrs = "", m = marker()) => `<${m}${name}${attrs ? " " + attrs : ""}>`
+
+/** A closing DSML tag. */
+export const closeTag = (name: string, m = marker()) => `</${m}${name}>`
+
+/** A parameter element. `string=false` encodes a JSON value. */
+export const param = (name: string, value: string, opts: { json?: boolean; m?: string; quote?: string } = {}) => {
+  const m = opts.m ?? marker()
+  const q = opts.quote ?? '"'
+  const stringAttr = opts.json ? ` string="false"` : ` string="true"`
+  return `${tag("parameter", `name=${q}${name}${q}${stringAttr}`, m)}${value}${closeTag("parameter", m)}`
+}
+
+/** An invoke element wrapping zero or more parameters. */
+export const invoke = (name: string, params: string[] = [], opts: { m?: string; quote?: string } = {}) => {
+  const m = opts.m ?? marker()
+  const q = opts.quote ?? '"'
+  return `${tag("invoke", `name=${q}${name}${q}`, m)}${params.join("")}${closeTag("invoke", m)}`
+}
+
+/** A tool_calls block wrapping one or more invokes. */
+export const block = (
+  invokes: string[],
+  opts: { m?: string; kind?: string } = {},
+) => {
+  const m = opts.m ?? marker()
+  const kind = opts.kind ?? "tool_calls"
+  return `${tag(kind, "", m)}${invokes.join("")}${closeTag(kind, m)}`
+}
+
+/** The end-of-sentence sentinel some backends emit. */
+export const eos = (bar = BAR) => `<${bar.repeat(2)}end\u2581of\u2581sentence${bar.repeat(2)}>`
+
+/** Concatenate fragments (makes intent explicit at call sites). */
+export const text = (...parts: string[]) => parts.join("")