fix: treat scalar observables as one observable

[danielgaskins]

Issue #, if available:

Closes #1341

Description of changes:

CircuitBinding now accepts a single Observable, PauliString, or Pauli word string and treats it as one observable. This keeps multi-qubit Pauli words intact and preserves signed Pauli strings.

ProgramSet.product() accepts the same scalar inputs. ProgramSet.zip() is unchanged because its observables are matched positionally to circuits.

Testing done:

• pytest -n 0 -q test/unit_tests/braket/program_sets
• ruff format --check src/braket/program_sets/circuit_binding.py src/braket/program_sets/program_set.py test/unit_tests/braket/program_sets/test_circuit_binding.py test/unit_tests/braket/program_sets/test_program_set.py
• ruff check src
• mypy

Merge Checklist

General

• I have read the contributing guide
• I used the required PR title format
• I updated the relevant type hints and API documentation

Tests

• I added tests that prove the fix works
• The tests do not depend on a specific region or account

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 (0da16fe) to head (d2ea760).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1343   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          171       171           
  Lines        11590     11594    +4     
  Branches      1517      1518    +1     
=========================================
+ Hits         11590     11594    +4     

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