✅ Add unit-level tests for `parseClassicConditionExpression`

[rturrado]

🤖 AI text below 🤖

Problem statement

parseClassicConditionExpression (in src/common/parsing/CodePreprocessing.cpp) has no direct unit tests. Every test that exercises it today lives in test/test_custom_code.cpp and runs the full end-to-end pipeline: load QASM code, spin up a DDSimulationState, run the simulation, and inspect amplitudes or classical bits.

Two problems with that coverage:

• A parser bug shows up as a failed amplitude assertion, not as "the parser returned the wrong ClassicCondition". Localizing the root cause takes longer than it should.
• Edge cases of the parser itself (malformed input, whitespace, leading paren, trailing garbage, invalid brackets, empty operand) are not systematically covered because writing a full QASM program to trigger each one is disproportionate.

Surfaced during the review of PR #463 while considering the follow-ups for parseClassicConditionExpression.

Proposed solution

Add a fixture (either in the existing test_parsing.cpp or in a new file) that calls parseClassicConditionExpression directly with a string and checks the returned std::optional<ClassicCondition> against expected values. Cover:

• Valid cases: one test per comparator (==, !=, <, <=, >, >=), bracketed forms (c[0] == 1), bare register (c), bare bit (c[0]), input with leading (, input with surrounding whitespace.
• Invalid cases: empty string, missing operand on either side, non-numeric right-hand side, malformed brackets (open only, close only, index empty, non-digit index, bracket not at the end), out-of-range integer literal.
• Document currently permissive behavior (e.g. c2] accepted as a register name, @# accepted). These tests can start green against the current behavior and flip to expect std::nullopt once 🐛 Register-reference validation is incomplete in the parser #468 lands.

Out of scope

• Unit tests for other parsers in the file (parseFunctionDefinition, parseClassicControlledGate, etc.). Same idea, separate issue.
• Testing parseBitRegisterRef directly. It lives in the anonymous namespace of the .cpp; the tests here cover it indirectly through parseClassicConditionExpression, which is what the outside world sees.