fix(miner): surface costUsd on the changed-files-enumeration-failure path too

Reconciling this branch with #5356 (costUsd wiring) surfaced a gap: the new
changed-file-enumeration-failure return statement ran after the SDK result
message (and its billed total_cost_usd) was already available, but omitted
costUsd -- silently undercounting real spend for that one failure path in
budgetSpent. Added it, plus regression coverage in both the node:test source
and its vitest/Codecov mirror.

Author: JSONbored

packages/gittensory-engine/test/agent-sdk-driver.test.ts

@@ -97,10 +97,10 @@ test("success derives changed files from the worktree after untracked mutating t
   assert.deepEqual(result.changedFiles, ["packages/gittensory-engine/src/vulnerable.ts"]);
 });
 
-test("success fails closed when changed-file enumeration is unavailable", async () => {
+test("success fails closed when changed-file enumeration is unavailable, but still reports the real dollar cost", async () => {
   const driver = driverWith({
     query: queryYielding([
-      { type: "result", subtype: "success", is_error: false, num_turns: 2, result: "done" },
+      { type: "result", subtype: "success", is_error: false, num_turns: 2, result: "done", total_cost_usd: 0.0042 },
     ]),
     listChangedFiles: async () => {
       throw new Error("not a git worktree");
@@ -112,6 +112,9 @@ test("success fails closed when changed-file enumeration is unavailable", async
   assert.equal(result.ok, false);
   assert.deepEqual(result.changedFiles, []);
   assert.match(result.error!, /agent_sdk_changed_files_unavailable: not a git worktree/);
+  // The SDK session ran and was billed before enumeration ever failed -- budgetSpent must not silently
+  // undercount this path just because the changed-files step failed afterward.
+  assert.equal(result.costUsd, 0.0042);
 });
 
 test("success stringifies a non-Error changed-file enumeration failure", async () => {

test/unit/agent-sdk-driver.test.ts

@@ -134,10 +134,10 @@ describe("createAgentSdkCodingAgentDriver", () => {
     expect(result.changedFiles).toEqual(["packages/gittensory-engine/src/vulnerable.ts"]);
   });
 
-  it("fails closed when changed-file enumeration is unavailable", async () => {
+  it("fails closed when changed-file enumeration is unavailable, but still reports the real dollar cost", async () => {
     const driver = driverWith({
       query: queryYielding([
-        { type: "result", subtype: "success", is_error: false, num_turns: 2, result: "done" },
+        { type: "result", subtype: "success", is_error: false, num_turns: 2, result: "done", total_cost_usd: 0.0042 },
       ]),
       listChangedFiles: async () => {
         throw new Error("not a git worktree");
@@ -149,6 +149,9 @@ describe("createAgentSdkCodingAgentDriver", () => {
     expect(result.ok).toBe(false);
     expect(result.changedFiles).toEqual([]);
     expect(result.error).toContain("agent_sdk_changed_files_unavailable: not a git worktree");
+    // The SDK session ran and was billed before enumeration ever failed -- budgetSpent must not silently
+    // undercount this path just because the changed-files step failed afterward.
+    expect(result.costUsd).toBe(0.0042);
   });
 
   it("stringifies a non-Error changed-file enumeration failure", async () => {