[Hybrid] Enable spec decoding in mamba cache align mode

[peakcrosser7]

Purpose

Re-enabled spec decoding (no async-scheduling): Previously disabled in #30877, speculative decoding is now re-enabled as the related issues have been confirmed as fixed.

Test Plan

pytest -vs tests/v1/e2e/test_mamba_prefix_cache.py

Test Result

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================== 1 passed, 10 warnings in 253.24s (0:04:13) ==================
sys:1: DeprecationWarning: builtin type swigvarlink has no __module__ attribute

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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.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request re-enables speculative decoding for Mamba models when using the 'align' cache mode. The change primarily consists of removing a check that previously disallowed this combination. A corresponding end-to-end test has also been re-enabled to validate the functionality. The changes are clear, well-targeted, and I have not found any issues.

[tdoublep]

LGTM

[mergify]

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