Clarify assignment-expression diagnostic in comprehension iterables

[Shinzzyak]

Fixes #11514.

Use the grammar-specific diagnostic when an assignment expression appears in a comprehension iterable, where adding parentheses cannot make it valid.

Tests: npm run build; npm run test:norebuild -- --runInBand parser.test.ts.

[Shinzzyak]

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[Shinzzyak]

@microsoft-github-policy-service agree

[rchiodo]

🔒 Automated review in progress — @rchiodo is auto-reviewing this PR.

[rchiodo]

📍 packages/pyright-internal/src/tests/parser.test.ts:203
The only new test uses a nested comprehension, while the issue's headline scenario is the direct iterable [x for x in (z := range(10))]. Add a second assertion on that canonical (parenthesized, still-rejected) form so the regression is tied to #11514's primary case and documents that parentheses do not help.

[rchiodo]

Clean, well-scoped fix. Consider also asserting the issue's canonical direct-iterable form to lock in the primary regression; otherwise looks good to merge.

[rchiodo]

Approved via Review Center.

[rchiodo]

Approved via Review Center.