Integrate MFSDP v2 with Megatron Bridge

[wujingyue]

Goal

Enable Megatron Bridge training recipes to use the experimental MFSDP v2 path.

Scope

Build on the Megatron Core integration from #5614 and add the Bridge-side configuration and recipe plumbing needed to select MFSDP v2.

Intended outcome

• Expose MFSDP v2 through an experimental Bridge configuration.
• Map the Bridge configuration to the MFSDP v2/MCore training path.
• Preserve existing FSDP behavior when MFSDP v2 is not selected.
• Add a minimal Bridge smoke test or recipe that verifies training can start with MFSDP v2 enabled.