feat: Add get_submetagraphs method for filtering by netuid (#2050)

[0xsid0703]

Summary

Implements get_submetagraphs(netuid) method that returns all submetagraphs (mechagraphs) for a specific netuid, addressing issue #2050.

Problem

Currently, get_all_mechagraphs() returns up to 128*8 items (all netuids × all mecids). When subnet owners enable many sub-subnets, this becomes inefficient when only data for one subnet is needed.

Solution

Add get_submetagraphs(netuid) that:

• Returns all mechagraphs for a single netuid
• Includes all sub_ids (mecids) for that netuid
• Returns empty vector if subnet doesn't exist
• More efficient than filtering get_all_mechagraphs() results

Changes

• ✅ Added get_submetagraphs() in pallet implementation
• ✅ Added runtime API trait method
• ✅ Added runtime implementation
• ✅ Added RPC endpoint subnetInfo_getSubmetagraphs
• ✅ Added 7 comprehensive unit tests

Testing

All unit tests pass, covering:

• Non-existent subnets
• Empty mechanisms
• Single and multiple mechanisms
• Filtering verification
• Comparison with get_all_mechagraphs()
• Registered neurons scenarios

Closes #2050

Contribution by Gittensor, learn more at https://gittensor.io/

[0xsid0703]

@gztensor @sam0x17 I’m happy to contribute to Subtensor. When you have a moment, could you review my PR?