docs(parser): add AGENTS.md test guidance for agents (#23440)

This PR adds an `AGENTS.md` to `crates/oxc_parser`. AI agents often try to inline unit tests tests for parser bugs which is incorrect. These tests should go inside the `tasks/coverage/misc` fixtures instead. Writing unit tests often doesn't test/snapshot the diagnostic which can cause regressions to go unnoticed if only the error count is being asserted

crates/oxc_parser/AGENTS.md

@@ -0,0 +1,43 @@
+# Coding agent guides for `crates/oxc_parser`
+
+## Testing parser behavior
+
+Parser syntax regressions should usually be tested through coverage fixtures, not Rust unit tests in this crate.
+
+Before adding a `misc` fixture, check whether the case is already covered by the normal `just coverage` suites. Do not duplicate coverage from Test262, Babel, TypeScript, or another existing fixture.
+
+Use:
+
+```text
+tasks/coverage/misc/pass/
+tasks/coverage/misc/fail/
+```
+
+Put valid syntax that must parse successfully in `pass/`, and invalid syntax or diagnostic regressions in `fail/`.
+
+If the regression fixes a tracked OXC issue, name the fixture after the issue. Otherwise, choose a short descriptive name for the syntax or diagnostic category:
+
+```text
+tasks/coverage/misc/pass/oxc-12345.ts
+tasks/coverage/misc/fail/oxc-12345.js
+tasks/coverage/misc/fail/missing-conditional-alternative.js
+```
+
+Run a focused parser coverage case with:
+
+```sh
+cargo coverage parser --filter misc/pass/oxc-12345.ts
+cargo coverage parser --filter misc/fail/oxc-12345.js
+```
+
+Run all parser coverage with:
+
+```sh
+cargo coverage parser
+```
+
+## When to use crate tests
+
+Use Rust tests in `crates/oxc_parser` only for parser API behavior, lexer internals, or focused implementation details that cannot be represented as a source fixture.
+
+Do not add ordinary syntax or diagnostic regressions there when a `tasks/coverage/misc` fixture can cover the behavior.