fix: treat a single OpenQASM string or Circuit as one ProgramSet entry

[alyabouzaid]

Problem

ProgramSet.__init__ wraps only CircuitBinding and Program in a list. A bare OpenQASM string is therefore treated as a sequence and iterated character by character, silently:

qasm = "OPENQASM 3.0;\nbit[2] b;\nqubit[2] q;\nh q[0];\ncnot q[0], q[1];\nb = measure q;"
ps = ProgramSet(qasm)
ps.total_executables        # 75  (== len(qasm))
ps.to_ir().programs[0]      # Program(source='O')

No error is raised; the malformed program set is only caught at the service. A bare Circuit hits the same line and raises TypeError: 'Circuit' object is not iterable.

Both types are already valid list entries, and a standalone Program is already supported (test_standalone_openqasm_program), so a standalone string or circuit is a natural thing to write.

Fix

Include Circuit and str in the isinstance check that wraps a single entry, and widen the type hint/docstring accordingly. ProgramSet(qasm) now equals ProgramSet([qasm]), and a single Circuit with result types now raises the intended ValueError: Circuit cannot have result types.

Verification

• Added test_standalone_openqasm_string and test_standalone_circuit, mirroring the existing test_standalone_openqasm_program. Both fail on main (75-character split / TypeError) and pass with the fix.
• pytest test/unit_tests: 3492 passed, 131 xfailed.
• ruff check src, ruff format --check ., mypy: clean.
• Exercised manually: entries, len, total_executables, total_shots, to_ir, enumerate_executables, split, __eq__, __add__ on a standalone string and a standalone circuit, plus the unchanged CircuitBinding / Program / mixed-list paths.

AI disclosure: this PR was prepared with the assistance of Claude Code (Anthropic's Claude); I review and test all changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3ff5f36) to head (9b27ae5).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1352   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          171       171           
  Lines        11648     11648           
  Branches      1538      1538           
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  Hits         11648     11648           

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[sesmart]

Simple fix, nice!