Add documentation and fix type safety bugs

ISSUE.md

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+# Bug report
+
+**Describe the bug**
+
+The recent refactor introduced a missing type safety and potential runtime error:
+- `remove_ctx` and `add_ctx` lacked documentation, making their purpose unclear.
+- `get_send_file_max_age` returned a value without proper type casting, triggering `type: ignore` comments.
+- The static file route used a lambda referencing a weakref; if the app was garbage‑collected it could raise an obscure error.
+- `raise_routing_exception` raised `request.routing_exception` without guaranteeing it was not `None`, leading to a possible `TypeError`.
+
+These issues manifested as type‑checking failures and potential crashes when serving static files or handling routing exceptions.
+
+**Steps to reproduce**
+
+1. Run the test suite (`pytest`).
+2. Observe `type: ignore` warnings and potential failures in `test_regression.py` when static files are accessed.
+3. Manually trigger a routing exception (e.g., abort with a redirect) and notice that `raise_routing_exception` may raise `None`.
+4. Access a static file after the app has been garbage‑collected (unlikely in normal use but possible in long‑running processes).
+
+**Expected behavior**
+
+- Functions should have clear docstrings.
+- `get_send_file_max_age` should return an `int` or `None` with proper type casting.
+- The static file view should raise a clear `RuntimeError` if the app is unavailable.
+- `raise_routing_exception` should assert the exception exists before raising.
+
+**Environment**
+
+- Python version: 3.12
+- Flask version: 3.2.0.dev
+
+---
+
+*This issue was created automatically using the repository's issue template.*