Add in and not-in expression operators (#170)

[Jeckerson]

Summary

Add support for in and not in (! in) as expression-level operators, enabling SQL-like membership tests in Zephir code.

Resolves #170
Related: phalcon/cphalcon#16812

Syntax

// Membership test
if x in [1, 2, 3] {
}

// Negated membership test
if x ! in [1, 2, 3] {
}

AST Output

• expr in expr produces a node with type "in"
• expr ! in expr produces a node with type "not-in"

Changes

• parser/zephir.lemon: Added IN to operator precedence (same level as INSTANCEOF). Added two new xx_common_expr grammar rules for in and not in.
• CHANGELOG.md: Updated unreleased section.
• tests/operators/in.phpt: Basic in and not in test.
• tests/operators/in-for-loop.phpt: Regression test ensuring in inside for loops still works and in as expression works within loop bodies.
• tests/operators/in-complex.phpt: Complex expressions with method calls, static constants, and parenthesized expressions.

Notes

• The existing in keyword for for loops is completely unaffected (0 parser conflicts).
• All 113 tests pass (110 existing + 3 new).
• The Zephir compiler will need a separate update to handle the new "in" and "not-in" AST node types — a companion issue will be created in zephir-lang/zephir.

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist.
• I wrote some tests for this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.24%. Comparing base (c2a6cd6) to head (f094983).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
+ Coverage   77.07%   77.24%   +0.17%     
==========================================
  Files           5        5              
  Lines        2992     2997       +5     
  Branches      131      131              
==========================================
+ Hits         2306     2315       +9     
+ Misses        686      682       -4     

Flag	Coverage Δ
Linux	77.24% <100.00%> (+0.17%)	⬆️
unittests	77.24% <100.00%> (+0.17%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Jeckerson]

Closing this PR.

After investigating the original cphalcon issue (phalcon/cphalcon#16812), the root cause is a string literal typo in phalcon/Mvc/Model/Query.zep line ~2093 where "BETWEEN NOT" should be "NOT BETWEEN". This is a one-line fix in cphalcon, not a Zephir parser issue.

The PHQL scanner/parser already handles NOT IN (PHQL_T_NOTIN) and NOT BETWEEN (PHQL_T_BETWEEN_NOT) correctly at the C level. Adding in/not in as Zephir language expression operators is not needed to resolve the bug.

See analysis comment: phalcon/cphalcon#16812 (comment)