[Bugfix][mamba] Fix type annotation of Mamba2Metadata

[heheda12345]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Fix some names and type annotation in Mamba2Metadata. All logic is unchanged. Prefer to get this PR in because people are copying these files to implement more mamba-like models in v1.

Test Plan

VLLM_USE_V1=1 HF_ALLOW_CODE_EVAL=1  lm_eval --model vllm \
    --model_args pretrained=mistralai/Mamba-Codestral-7B-v0.1,enable_prefix_caching=False \
    --tasks humaneval \
    --device cuda:0 \
    --batch_size auto \
    --confirm_run_unsafe_code

Test Result

match the previous result.

|  Tasks  |Version|  Filter   |n-shot|Metric|   |Value |   |Stderr|
|---------|------:|-----------|-----:|------|---|-----:|---|-----:|
|humaneval|      1|create_test|     0|pass@1|   |0.4024|±  |0.0384|

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request correctly fixes several type annotation issues in Mamba2AttentionMetadata by renaming some fields to be more specific to prefill operations and making them optional. The changes improve the clarity and correctness of the type hints. I've found one more minor type annotation issue in the same file that you might want to address as well.

[heheda12345]

CC @tlrmchlsmth @tdoublep

[Josephasafg]

LGTM

[tdoublep]

LGTM - I've been confused by which of these fields are used for what purpose in the past, and this will help next time.