Feature Request: Pull-Based Diagnostics (LSP 3.17)

Description

The Angular Language Service should implement Pull-Based Diagnostics (LSP 3.17) to allow VS Code to request diagnostics on-demand rather than having the server push them on every change.

Current Behavior

Currently, the Angular Language Service uses push-based diagnostics via textDocument/publishDiagnostics. The server pushes diagnostics to the client on every file change, which can cause:

1. Redundant computations: Diagnostics are recomputed even for unchanged files
2. Server-driven timing: No control over when diagnostics are computed
3. Unnecessary updates: Switching between tabs triggers diagnostic recomputation

Expected Behavior

Pull-Based Diagnostics (LSP 3.17) should:

1. Cache results: Return Unchanged report in ~0.2ms when file hasn't changed
2. Client-controlled timing: Let VS Code decide when to request diagnostics
3. Workspace batching: Support workspace/diagnostic for all open files
4. Refresh signal: Support workspace/diagnostic/refresh for server-initiated refresh

Protocol Flow

Client                          Server
  |                                |
  |-- textDocument/diagnostic ---->|
  |      (with previousResultId)   |
  |                                |
  |<-- Unchanged (resultId match)--|  <- Cache hit: ~0.2ms
  |         OR                     |
  |<-- Full (new diagnostics) ----|  <- Cache miss: compute

Motivation

This feature works together with #66668 (Client-Side File Watching) to provide comprehensive performance improvements:

Implementation Notes

• Requires LSP 3.17 support (vscode-languageserver v9.0.1+)
• Should fall back to push-based diagnostics for older clients
• Cache should be keyed by resultId and document version

Labels

• area: language-service
• feature
• vscode-extension