[Cisco] Fix FBOSS mirror DSCP encoding

[peterpiao27]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

clang-format.............................................................Passed
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trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
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check for merge conflicts................................................Passed
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Prevent sai_impl in fboss manifest.......................................Passed

Summary

FBOSS passed the configured 6-bit mirror DSCP directly to the SAI 8-bit TOS attribute. This caused DSCP 42 to produce an on-wire DSCP of 10.

Convert DSCP to TOS using DSCP << 2 for sFlow and ERSPAN mirror sessions, and update the associated mirror verification tests.

Test Plan

• Built fboss-forwarding-stack successfully.
• Deployed the updated HW agent on fboss dut.
• Verified through the SAI replay log that configured DSCP 42 programs TOS 168 (0xA8).
• Verified the dut sFlow mirror is active and resolved.
• Verified in Ixia packets capture that exported sFlow packets have Traffic Class 0xA8, corresponding to DSCP 42 and ECN 0.