Refactor _AnnotationMapper (#9020)

This has two main goals:

1. Move all validation that does not depend on the task we're annotating
   from `_AnnotationMapper` and to `DetectorFunctionSpec`. This a) splits
   the code into more manageable chunks, and b) works better with the agent
   workflow, because the CLI `function` commands can reject an invalid
   function spec immediately (rather than waiting until the first request).

   (Frankly, I don't know why I didn't put this code in
   `DetectorFunctionSpec` in the first place, given that it implements the
   restrictions described in the docstring...)

   Validating the spec upon construction requires that the spec is not
   modified afterwards. This has always been my intention, but to make it
   clearer, state it explicitly in the docstring.

2. Restructure the rest of the code into smaller functions. This should
   not introduce any differences in behavior.

changelog.d/20250130_123343_roman_refactor_annotation_mapper.md

@@ -0,0 +1,5 @@
+### Changed
+
+- `DetectorFunctionSpec` will now raise `BadFunctionError` if it detects
+  any violations of the documented constraints on the labels
+  (<https://github.com/cvat-ai/cvat/pull/9020>)

cvat-sdk/cvat_sdk/auto_annotation/__init__.py

@@ -2,7 +2,8 @@
 #
 # SPDX-License-Identifier: MIT
 
-from .driver import BadFunctionError, annotate_task
+from .driver import annotate_task
+from .exceptions import BadFunctionError
 from .interface import (
     DetectionFunction,
     DetectionFunctionContext,