feature: add Circuit instruction counting

[baihaoyu11]

Issue #, if available: Fixes #1235

Description of changes:
Adds Circuit.count(instruction_name: str | None = None) so callers can count all instructions in a circuit or count instructions matching an operator name case-insensitively.

Testing done:

• .venv/bin/python -m pytest test/unit_tests/braket/circuits/test_circuit.py -k 'count' (10 passed)
• .venv/bin/python -m pytest test/unit_tests/braket/circuits/test_circuit.py (338 passed, 2 xfailed)
• .venv/bin/python -m ruff format --check src/braket/circuits/circuit.py test/unit_tests/braket/circuits/test_circuit.py
• .venv/bin/python -m ruff check src/braket/circuits/circuit.py

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