Ensure that last prefill chunk is handled correctly by Mamba models

[santhnm2]

What does this PR do ?

This PR fixes a scheduler bug for chunked prefill with Mamba models. The scheduler sets chunked_prefill_request_id to -1 for the last prefill chunk and then continues scheduling additional requests if it can. This PR instead terminates scheduling early for Mamba models on the last prefill chunk and also sets the has_explicit_chunked_prefill_req flag to True for any prefill chunk, including the last one. This ensures that Mamba models always go through the chunked prefill path when they need to. This PR also adds a functional test for Mamba chunked prefill and disables chunked prefill by default.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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[santhnm2]

/ok to test 7a2ac67

[santhnm2]

/ok to test 0e96a3e

[santhnm2]

/ok to test 242f17a

[santhnm2]

/ok to test e2d917a

[santhnm2]

/ok to test 454060e