Add an opt-in for capturing sanitizer logs from the language server

[sean-mcmanus]

Fix by Copilot with Claude Opus 4.8 in VS Code.

Problem

When running a sanitizer build of the language server (a -tsan/-asan/-ubsan
CMake preset), the sanitizer prints its reports to stderr. Because the
extension talks to the language server over stdio, those reports are easily
lost, and a crash log only shows the sanitizer shutting down, not the actual
report. There was no convenient, checked-in way to capture them, so developers
resorted to attaching a debugger (which can hang the process at shutdown) or
hand-editing the spawn code.

Change

Add an opt-in, gated on the CPPTOOLS_SANITIZER_LOG_DIR environment variable:

• client.ts: new getSanitizerServerEnv() helper. When CPPTOOLS_SANITIZER_LOG_DIR
  is set, it routes each sanitizer's log_path into that directory and returns the
  merged env for the spawned language server; the cpptools-srv/-srv2 children
  inherit it, so every process writes its own "/." file
  (tsan/asan/ubsan). Any TSAN_OPTIONS/ASAN_OPTIONS/UBSAN_OPTIONS the developer
  already set are preserved. The helper best-effort creates the directory
  (the sanitizer runtime does not create a missing one; without it reports
  silently fall back to stderr). It is wired into both the run and debug
  ServerOptions.
• When the variable is unset (normal development, CI, released builds), the
  helper returns undefined, leaving the child environment inherited unchanged --
  a true no-op, safe to leave checked in.
• .vscode/launch.json: new "Run Extension (capture sanitizer logs)"
  configuration (based on "Run Extension") that sets
  CPPTOOLS_SANITIZER_LOG_DIR=${userHome}/cpptools-sanitizer-logs, so a developer
  can just pick it from the Run and Debug dropdown.
• readme.developer.md: new "Capturing sanitizer diagnostics from the language
  server" section (with a Table of Contents entry) documenting the variable, the
  launch configuration, and how to read the resulting files.

Usage

Build a sanitizer preset of the language server, pick "Run Extension (capture
sanitizer logs)" from the Run and Debug dropdown (or set
CPPTOOLS_SANITIZER_LOG_DIR yourself), reproduce, then read
~/cpptools-sanitizer-logs/tsan. (or asan.* / ubsan.*). No debugger is
required, which also avoids the shutdown-time hang that can occur when a
debugger is attached to a sanitizer process as it exits.

Notes

• This is in the vscode-cpptools (extension) repo; the native sanitizer/crash
  handler changes are a separate PR in the VS repo.
• No behavior change unless the environment variable is set.

Testing

Ran a TSan build of the language server with the new launch configuration;
reports were written to ~/cpptools-sanitizer-logs/tsan.. With the variable
unset, the spawned environment is unchanged and there is no behavior difference.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[sean-mcmanus]

I just made a ReadMe change corresponding to a PR I have in draft, so I'll wait for that one to be ready to check in.

UPDATE: Actually, that's a minor change, so it doesn't matter.