feature: count circuit instructions

[jamilahmadzai]

Issue #, if available:
Closes #1235

Description of changes:
Adds Circuit.count() for instruction operator counts. It returns the case-insensitive count for a requested operator, for example circuit.count("cnot"), and returns counts for all operators when no operator is provided. It can also limit counts to instructions touching a requested target qubit set.

Testing done:

• uv run --extra test python -m pytest -n 0 test/unit_tests/braket/circuits/test_circuit.py
• uv run --with ruff ruff check src/braket/circuits/circuit.py
• git diff --check

unitaryHACK:

• Registered GitHub user: @jamilahmadzai
• Payout: unitaryHACK event registration for @jamilahmadzai; no PR-specific public wallet/payment rail is requested in the issue.
• Tooling disclosure: LLM assistance was used while preparing this patch; the final diff and validation are included above.

Merge Checklist

General

• I have read the CONTRIBUTING doc
• I used the PR title format described in CONTRIBUTING
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a5ee47d) to head (aad5c85).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1287   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          169       169           
  Lines        10963     10969    +6     
  Branches      1412      1413    +1     
=========================================
+ Hits         10963     10969    +6     

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