fix: clamp format result range to document bounds (#153)

[maruthang]

Summary

Fixes #153

Bug: format could return a TextEdit with a range end exceeding the document length, causing downstream consumers to fail.

Root Cause: The formatter used the initial full-document range (lines 0 to MAX_VALUE) without clamping it back to actual document bounds before returning, and also returned a redundant edit when formatting produced no changes.

Fix: Added a no-op optimization that returns an empty array when the formatted result equals the original text, and added defensive range clamping that limits the edit range end to the document's actual end position.

Changes

• src/services/htmlFormatter.ts: Return early with [] when formatted output matches input; clamp range.end to document.positionAt(text.length) before constructing the returned TextEdit.
• src/test/formatter.test.ts: Added 6 regression tests covering empty documents, whitespace-only, newline-only, CRLF-only, endWithNewline on empty input, and already-formatted content — all asserting edit ranges stay within document bounds.

Testing

• Added regression tests in src/test/formatter.test.ts that verify all returned edit ranges do not exceed document length
• All 161 existing tests pass