fixtures: deprecate calling request.getfixturevalue() during teardown

[bluetech]

on a fixture that hasn't already been requested.

Fix #12882.

The first commit is needed for the $subject change.

[themavik]

Deprecates calling request.getfixturevalue() during teardown for fixtures not already requested. Addresses #12882.

Done well: Clear deprecation docs explaining why (teardown runs during scope unwind, lookup order is brittle). Good test coverage: previously-requested OK, new fixture warns, inactive fixture errors. is_node_active is a clean abstraction. _raise_teardown_lookup_error gives a helpful message.

Concern: In formatrepr, the change from assert self.msg is None or self.fixturestack to if msg is not None and len(stack) > 1 removes the assert. The old assert guarded against empty fixturestack with non-None msg. The new logic avoids the crash by not using stack[:-1] when len(stack) <= 1. Worth a quick check that no code path can hit msg is not None with empty fixturestack—if so, the formatrepr output might be odd. Otherwise looks good.