poc(XCM): Deny Nested XCM Barriers

[raymondkfcheung]

Resolves (partially): #7148
Depends on: #7169, #7200

Description

For context and additional information, please refer to #7148 and #7200.

TODOs

• Rebase xcm: fix for DenyThenTry Barrier #7169 and XCM: Deny barrier checks for nested XCMs with specific instructions to be executed on the local chain #7200
• Evaluate PoC described on XCM: Deny barrier checks for nested XCMs with specific instructions to be executed on the local chain #7200

POC	Top-Level Denial	Nested Denial	Try Allow	Remark
DenyThenTry	✅	❌	✅	Blocks top-level instructions only.
RecursiveDenyThenTry	✅	✅	✅	Blocks both top-level and nested instructions.
DenyInstructionsWithXcm	❌	✅	❌	Focuses on nested instructions, requires additional checks for top-level denial.
DenyFirstInstructionsWithXcm	✅	✅	❌	Prioritises top-level denial before recursive checks.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/13070363687
Failed job name: fmt