Enable proto_mode handling code

[divyachandralekha]

What was added

• Introduced logic to handle proto_mode in STP processing: distinguishing between modes like L2_PVSTP, L2_MSTP, etc.
• In mstp.c, changed sequence of conditionals in mstp_updtRolesCist() and other MSTP-branching code.
• In stp_intf.c and stp_mgr.c, re-enabled or reorganized branches that were previously commented out (for example, handling of msg->proto_mode).
• In bitmap.c and bitmap.h, adjusted data structures or handling to support new mode logic (e.g., changes in how bitmaps are allocated or referenced).
• In include/stp.h, modified struct definitions or macros to include proto_mode as a field (or variant) to make mode-aware processing possible.

Verified

• Ensure proto_mode is correctly set in incoming messages and used in processing (for STP_BRIDGE_CONFIG, STP_PORT_CONFIG, STP_MST_VLAN_PORT_LIST_CONFIG, etc.).
• Confirm no regressions in PVST functionality while MSTP mode is added/enabled.
• Verify memory/bitmap operations did not break after changes in bitmap.c/h (allocation, count bits, print, etc.).
• Validate build, unit tests for PVST and MSTP

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[divyachandralekha]

@skulambi-cisco , Please review and approve

[divyachandralekha]

@muhammadalihussnain Please review

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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