[Cohere] Cohere2 moe parser fix

[Terrencezzj]

Purpose

• Normalize Cohere Command streaming tool-call deltas so Melody placeholder fields are omitted from OpenAI-compatible responses.
• Preserve pending tool-call IDs until the first real function-name delta, preventing clients from seeing invalid empty tool names.
• Update Cohere2 MoE weight loading to use the current fused_moe_make_expert_params_mapping helper after the FusedMoE API change.

Test Plan

Test Result

Before fix:

tool_delta_1: [{"id":"0","type":"function","index":0,"function":{"name":"","arguments":""}}]
tool_delta_2: [{"id":"","type":"function","index":0,"function":{"name":"bash","arguments":""}}]
tool_delta_3: [{"id":"","type":"function","index":0,"function":{"name":"","arguments":"{\""}}]
tool_delta_4: [{"id":"","type":"function","index":0,"function":{"name":"","arguments":"command"}}]
tool_delta_5: [{"id":"","type":"function","index":0,"function":{"name":"","arguments":"\":"}}]
tool_delta_6: [{"id":"","type":"function","index":0,"function":{"name":"","arguments":" \"pwd\"}"}}]

After fix

tool_delta_1: [{"id":"0","type":"function","index":0,"function":{"name":"bash"}}]
tool_delta_2: [{"index":0,"function":{"arguments":"{\""}}]
tool_delta_3: [{"index":0,"function":{"arguments":"command"}}]
tool_delta_4: [{"index":0,"function":{"arguments":"\":"}}]
tool_delta_5: [{"index":0,"function":{"arguments":" \"pwd\"}"}}]

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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.

[sfeng33]

Is this needed for tool parser fix

[Terrencezzj]

No, but it breaks our model. The PR caused this was merged this morning #41184

[sfeng33]

Thanks!