Update buffer parameters for AgentIngressBufferTest on TH6

Summary:
cold_boot.AgentIngressBufferTest.validateConfig - this test was failing for TH6

Update buffer configuration parameters for Tomahawk6 ASIC to fix AgentIngressBufferTest failures.

- Changed MMU cell size from 254 to 420 bytes in Tomahawk6Asic to match actual hardware
- Increased pool reserved cells from 200 to 4000 to provide sufficient buffer resources

These changes address SAI_STATUS_INSUFFICIENT_RESOURCES errors when assigning buffer profiles to ingress priority groups.

Reviewed By: msomasundaran

Differential Revision: D91076142

fbshipit-source-id: 355b150df0e9f367260ce658db8bc3cfffe67359

Author: Prashasthi Melanta

fboss/agent/test/agent_hw_tests/AgentIngressBufferTests.cpp

@@ -23,7 +23,7 @@ constexpr int kPgMinLimitCells = 6;
 constexpr int kPgHeadroomLimitCells = 2;
 constexpr int kPoolHeadroomLimitCells = 10;
 constexpr int kPoolSharedCells = 10000;
-constexpr int kPoolReservedCells = 200;
+constexpr int kPoolReservedCells = 4000;
 /*
  * SDK has the expectation that pool_total_size - delta <= shared size,
  * where delta is the difference between old and new pool limits, if