[BugFix][KVCache] Add enc_dec_block_num to prefill_kvcache_block_num check

[kevincheng2]

Motivation

Cherry-pick from release/2.5: the original assertion only checked
prefill_kvcache_block_num >= max_block_num_per_seq, but for
encoder-decoder models the kvcache must also reserve blocks for the
encoder side (enc_dec_block_num). Without this check, the service
could silently allocate insufficient blocks for enc-dec sequences.

Modifications

• CacheConfig.postprocess: tighten assertion to
  prefill_kvcache_block_num >= max_block_num_per_seq + enc_dec_block_num,
  error message guides user to reduce max_model_len or increase
  num_gpu_blocks_override
• CacheConfig.reset: same tightening, error message guides user to
  reduce max_model_len or replace with larger GPU cards (override
  is not applicable here)

Usage or Command

No change to launch command. If the assertion fires, adjust:

# Option 1: reduce max_model_len
python -m fastdeploy.entrypoints.openai.api_server \
  --max-model-len <smaller_value> ...

# Option 2 (postprocess only): increase GPU block count
python -m fastdeploy.entrypoints.openai.api_server \
  --num-gpu-blocks-override <larger_value> ...

Checklist

• Add at least a tag in the PR title.
• Format your code, run pre-commit before commit.
• Add unit tests. No new logic introduced, only assertion tightened.
• Provide accuracy results. Not applicable.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[kevincheng2]

Closing to resubmit with a clean branch.