[refactor]: remove VideoGenerator.generate_video

VideoGenerator.generate(request) is the one inference entry point. The
generate_video keyword call, its mapping from flat keywords to a request
(legacy_generate_call_to_request and its helpers), and the tests of that
mapping are removed. Comments and docs that named generate_video describe
generate and request extensions instead.

scripts/ltx2_sr_alignment.py keeps its generate_video call, because its
reference run imports the separate FastVideo-internal package.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012M91wnVFmPEvJ9h39r5BH7

fastvideo/api/sampling_param.py

@@ -140,7 +140,7 @@ class SamplingParam:
     ltx2_stg_blocks_audio: list[int] = field(default_factory=lambda: [29])
 
     # LTX-2 image / video / continuation conditioning. These flow from
-    # generate_video(...) kwargs through ``sampling_param.update(kwargs)``
+    # request ``extensions`` through ``sampling_param.update(...)``
     # onto the ForwardBatch fields of the same name. ``ltx2_image_crf``
     # gates the conditioning-image H.264 re-encode; the streaming
     # session controller passes ``ltx2_image_crf=0.0`` because it