Fix error message truncation in TypeScript validator

[Copilot]

TypeScript diagnostic code 2740 (triggered when 6+ required properties are missing) hardcodes props.slice(0, 4) internally, producing truncated messages like:

Type '{ id: number; title: string; }' is missing the following properties from type 'Doc': slug, abstract, description, and 4 more.

Additionally, the original code extracted only the top-level .messageText from a DiagnosticMessageChain, silently dropping any nested detail in the .next chain.

Changes (typescript/src/ts/validate.ts)

• DiagnosticMessageChain handling: Replace manual d.messageText.messageText extraction with ts.flattenDiagnosticMessageText(d.messageText, "\n"), which correctly traverses the full chain.

• expandMissingPropertiesMessage helper: For diagnostic code 2740, uses program.getTypeChecker() to reconstruct the complete missing-property list. Locates the exact variable declaration via decl.getStart(file) <= d.start <= decl.end, filters optional symbols (ts.SymbolFlags.Optional), and formats with ts.TypeFormatFlags.NoTruncation. Falls back to the original compiler message if expansion fails.

// Before
Type '{ id: number; title: string; }' is missing the following properties
from type 'Doc': slug, abstract, description, and 4 more.

// After
Type '{ id: number; title: string; }' is missing the following properties
from type 'Doc': slug, abstract, description, author, date, category, content, tags

Note: noErrorTruncation compiler option does not address this — it only affects type name length, not the property list count.

Tests (typescript/test/validate.test.ts)

Added 12 unit tests using Node.js's built-in node:test / node:assert (no new dependencies), runnable via npm test. The tests verify:

• Valid objects continue to pass validation (backward-compatible)
• Type-mismatch errors (TS codes other than 2740) are unaffected
• Single missing property (TS 2741) and 2–5 missing properties (TS 2739) are unaffected
• 6+ missing required properties (TS 2740) now returns the full property list with no "and N more" truncation
• Optional properties are correctly excluded from the missing-properties list