feat: add Circuit instruction counting

[yanziwei]

Summary

• add Circuit.count() to count instructions by lowercase operator name
• support Circuit.count("cnot") for a single instruction type and Circuit.count() for all instruction counts
• add regression coverage for case-insensitive lookup, missing instructions, empty circuits, and ignoring result types

Closes #1235.

Validation

• .venv/bin/python -m pytest test/unit_tests/braket/circuits/test_circuit.py -q
• .venv/bin/python -m ruff check src/braket/circuits/circuit.py
• .venv/bin/python -m ruff format --check src/braket/circuits/circuit.py test/unit_tests/braket/circuits/test_circuit.py
• git diff --check

Note: running ruff check on the full existing test_circuit.py with the latest local ruff surfaces many pre-existing findings unrelated to this PR, so I scoped lint to the changed source file and used format checking for both touched files.

AI use

I used an AI coding assistant to inspect the existing circuit API, implement the small method and tests, and run validation. I reviewed the final diff before submission.

[veena-vee989]

@yanziwei (cc @sesmart)

Hello - you are currently over the PR limit for unitaryHACK, so your PRs will be disregarded until you align with the rules. Reach out to hack@unitary.foundation to be reinstated. (PR 2 of 7)