Fix LSP panic on negative position coordinates

[JRI98]

LSP request handlers panicked when receiving negative line or character numbers in request position parameters due to unvalidated integer conversion to unsigned types.

This was addressed by validating incoming position line and character integers across LSP request handlers, returning an invalid parameters error or handling out-of-range values gracefully rather than crashing.

Fixes #10854

[greptile-apps]

Greptile Summary

The PR prevents malformed LSP position coordinates from reaching unsigned position operations.

• Uses checked u32 conversion in completion, definition, hover, document-highlight, and selection-range handlers.
• Returns invalid_params for negative, oversized, missing, or mistyped coordinates instead of panicking or targeting document start.
• Adds regression tests for negative coordinates across all affected handlers.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/lsp/handlers/document_highlight.zig	Replaces fallback-to-zero parsing with explicit validation and checked coordinate conversion, fully addressing the prior finding.
src/lsp/handlers/selection_range.zig	Validates every requested position and safely abandons accumulated results before returning one invalid-parameters response.
src/lsp/handlers/completion.zig	Rejects coordinates outside the valid u32 range before document-position processing.
src/lsp/handlers/definition.zig	Uses checked coordinate conversion and returns an invalid-parameters response on conversion failure.
src/lsp/handlers/hover.zig	Uses checked coordinate conversion to prevent negative or oversized values from reaching lookup logic.
src/lsp/test/handler_unit_tests.zig	Adds regression coverage confirming negative coordinates produce invalid-parameters errors without panicking.

_{Reviews (2): Last reviewed commit: "Fix LSP panic on negative position coord..." | Re-trigger Greptile}