[Routine - VT] fix(mcp): guard files iteration in validate_json_structure against undefined

[winterdrive]

1. Pre-flight Check

Open PRs checked (all Draft, all CLEAN merge status):

• [Routine - VT] fix(AutoGrouper): move bookmarks to sub-groups on extension/date auto-grouping #81 routine/vt-fix-autogroup-bookmarks-260708 — touches src/core/AutoGrouper.ts, src/test/unit/autoGrouperBookmarks.test.ts
• [Routine - VT] fix(commands): clamp bookmark position to document bounds in jumpToBookmark #80 routine/vt-fix-jumptobookmark-clamp-260707 — touches src/commands.ts
• [Routine - VT] fix(FileManager): use normalized membership check for add/remove files #79 routine/vt-fix-filemanager-relpath-260706 — touches src/core/FileManager.ts, src/test/unit/fileManagerRelpath.test.ts
• [Routine - VT] fix(provider): guard against stale groupIdx when rendering file bookmarks #78 routine/vt-fix-bookmark-stale-groupidx-260703 — touches src/provider.ts
• [Routine - VT] fix(extension): dispose treeView listeners and pending reveal timer #77 routine/vt-fix-treeview-listener-disposal-260702 — touches src/extension.ts
• chore: release v0.7.6 #76 release/0.7.6-260701 — release PR (non-code-diff concern)

This PR touches only mcp-server/src/server.ts, which is not modified by any of the above. No overlap.

2. Changes

validate_json_structure (the MCP tool AI agents are told to call before any direct write to .vscode/virtualTab.json) validates each group's optional files field with:

if (g.files !== undefined && !Array.isArray(g.files)) {
  errors.push(...);
} else {
  for (const f of g.files) { ... }   // BUG: runs even when g.files is undefined
}

Since files is a legitimately optional field (per the tool's own JSON schema), any group without a files key hits the else branch and does for (const f of undefined), throwing TypeError: undefined is not iterable. That exception is swallowed by the handler's outer try/catch and reported back to the AI agent as "JSON parse error: ... is not iterable" — a misleading false negative for perfectly valid, syntactically correct JSON.

Fix: only iterate g.files when it is actually an array (else if (Array.isArray(g.files))), matching the same predicate already used for the invalid-type check.

Confirms this PR does not modify commands, contributed configuration keys, dependencies, or the tab-group serialization/storage format. No package.json/package-lock.json/CHANGELOG.md changes.

3. Safety Verification

Commands run locally, all passed:

• npx tsc -p ./ — no errors
• npx tsc -p ./mcp-server — no errors
• npm run test (tsc -p ./ && jest --runInBand) — 25 suites / 175 tests passed
• npm run build:mcp (esbuild bundle sanity check) — succeeded
• Manual repro: extracted the validation logic and ran it against [{ "id": "g1", "name": "Empty Group" }] (no files key) — before the fix this throws; after the fix it returns { "valid": true, "errors": [], "warnings": [] }

No lint or format:check npm scripts exist in this repo, so those steps were skipped per the routine-task instructions.

4. CI / Release Gate Note

This is a daily routine Draft PR. Package version bump and CHANGELOG.md consolidation are intentionally deferred to the weekend release/integration PR. If CI fails solely due to the repository's version-bump release gate, that is expected release-readiness behavior, not a code validation failure.