testbot: require go_test BUILD targets for new Go test files

src/scripts/testbot/TESTBOT_PROMPT.md

@@ -29,10 +29,28 @@ The targets are appended below this prompt. For each target you receive:
    - Target: boundary values, empty/None inputs, error paths, off-by-one.
 5. Write (or extend) the test file targeting the uncovered line ranges.
    Place new test files in the same location convention as step 2.
-6. **Python only — BUILD file**:
-   - Check the `BUILD` file in the test directory for an existing `py_test()` entry.
-   - If missing, add a `py_test()` rule. Infer `deps` from other `py_test` entries
-     in the same BUILD file. Do NOT guess target names.
+6. **BUILD file** (Python and Go — TypeScript uses Vitest discovery, no BUILD edit):
+   - **Python**: check the `BUILD` file in the test directory for an existing
+     `py_test()` entry. If missing, add a `py_test()` rule. Infer `deps` from
+     other `py_test` entries in the same BUILD file. Do NOT guess target names.
+   - **Go**: the test file lives next to the source (`<name>_test.go`), so
+     check the source package's `BUILD` for an existing `go_test()` entry. If
+     the BUILD only has `go_library` and you added a `_test.go`, you MUST also
+     add a `go_test` rule referencing it. Without the rule the test file is
+     invisible to Bazel and `bazel coverage` will not run it. Pattern (verified
+     against `src/runtime/pkg/data/BUILD`):
+     ```bzl
+     load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+     go_test(
+         name = "<pkg>_test",
+         srcs = ["<name>_test.go"],
+         embed = [":<pkg>"],   # the existing go_library target name
+         # add deps only if your test imports something the library doesn't
+     )
+     ```
+     Confirm the rule is loaded by running `bazel test <target>`; if you get
+     `ERROR: No test targets were found, yet testing was requested`, the
+     `go_test` rule is missing or misnamed.
 7. Run the test and verify code style per TESTBOT_RULES.md.
    If the test fails, follow the bug detection steps in TESTBOT_RULES.md.
 8. Move to the next target.
@@ -41,7 +59,8 @@ The targets are appended below this prompt. For each target you receive:
 
 - **Test files only**: You may ONLY create or modify test files (`test_*.py`,
   `*_test.go`, `*.test.ts`, `*.test.tsx`) and `BUILD` files (for `py_test`
-  entries). Do NOT modify source code, configuration, or other non-test files.
+  and `go_test` entries). Do NOT modify source code, configuration, or other
+  non-test files.
 - **No git or gh commands**: Do NOT run `git`, `gh`, or any commands that
   modify version control state. The harness script handles branch creation,
   committing, pushing, and PR creation.

src/scripts/testbot/TESTBOT_RULES.md

@@ -53,7 +53,10 @@ When a test fails:
 > see both the failure and the output. Just run the command directly.
 
 1. **Run the test**:
-   - Python/Go: `bazel test <target>` (derive the Bazel target from the BUILD file)
+   - Python/Go: `bazel test <target>` (derive the Bazel target from the BUILD file).
+     If `bazel test` reports `ERROR: No test targets were found, yet testing
+     was requested`, the test file isn't wired into BUILD — see "BUILD wiring"
+     below before retrying.
    - TypeScript: `pnpm --dir src/ui test -- --run <test_file_path>`
 2. If the test fails, read the error and fix. Retry up to 3 times.
 3. **Verify code style** (same checks as PR CI). Fix and re-verify until clean:
@@ -63,6 +66,19 @@ When a test fails:
      `pnpm --dir src/ui format` to auto-fix.
    - Go: no additional checks beyond step 1.
 
+### BUILD wiring (Python + Go)
+
+A test file that isn't registered as a Bazel target is invisible to CI —
+`bazel coverage //...` won't run it and Codecov won't see the new coverage,
+defeating the whole point of the testbot run.
+
+- **Python** (`test_<name>.py`): the `BUILD` file in the test directory must
+  have a `py_test()` rule whose `srcs` includes the new file.
+- **Go** (`<name>_test.go`): the source package's `BUILD` file must load
+  `go_test` and declare a `go_test()` rule pointing at the new file. A
+  `go_library`-only BUILD file silently drops adjacent `_test.go` files
+  on the floor.
+
 ## Language Conventions
 
 ### Python