fix(python): use PEP 508 string containment for in/not in operators

The `in` and `not in` marker operators were incorrectly splitting by
comma and checking array membership. Per PEP 508, these operators
perform Python string containment (e.g. `'linux' in 'linux2'` is true).

TC-4313

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ruromero

src/providers/marker_evaluator.js

@@ -88,8 +88,8 @@ function evaluateComparison(variable, op, value, env) {
 	switch (op) {
 	case '==': return envVal === value
 	case '!=': return envVal !== value
-	case 'in': return value.split(',').map(s => s.trim()).includes(envVal)
-	case 'not in': return !value.split(',').map(s => s.trim()).includes(envVal)
+	case 'in': return value.includes(envVal)
+	case 'not in': return !value.includes(envVal)
 	default: return envVal === value
 	}
 }

test/providers/marker_evaluator.test.js

@@ -0,0 +1,47 @@
+import { expect } from 'chai'
+
+import { evaluateMarker } from '../../src/providers/marker_evaluator.js'
+
+let originalPlatform
+
+suite('PEP 508 marker evaluator', () => {
+	suiteSetup(() => {
+		originalPlatform = process.platform
+		Object.defineProperty(process, 'platform', { value: 'linux', configurable: true })
+	})
+	suiteTeardown(() => {
+		Object.defineProperty(process, 'platform', { value: originalPlatform, configurable: true })
+	})
+
+	suite('in operator — PEP 508 string containment', () => {
+		test('substring match: sys_platform in wider string', () => {
+			expect(evaluateMarker("sys_platform in 'linux2'")).to.be.true
+		})
+
+		test('exact match: sys_platform in exact string', () => {
+			expect(evaluateMarker("sys_platform in 'linux'")).to.be.true
+		})
+
+		test('no match: sys_platform in unrelated string', () => {
+			expect(evaluateMarker("sys_platform in 'win32'")).to.be.false
+		})
+
+		test('partial overlap but no containment', () => {
+			expect(evaluateMarker("sys_platform in 'lin'")).to.be.false
+		})
+	})
+
+	suite('not in operator — negated string containment', () => {
+		test('substring present: sys_platform not in wider string', () => {
+			expect(evaluateMarker("sys_platform not in 'linux2'")).to.be.false
+		})
+
+		test('no match: sys_platform not in unrelated string', () => {
+			expect(evaluateMarker("sys_platform not in 'win32'")).to.be.true
+		})
+
+		test('partial overlap negated', () => {
+			expect(evaluateMarker("sys_platform not in 'lin'")).to.be.true
+		})
+	})
+})