Add configurable LSP backend support with rust-jdt-ls e2e test runner

[Copilot]

The extension was hardwired to the bundled Eclipse JDT LS with no mechanism to substitute an alternative LSP implementation (e.g. rust-jdt-ls).

New configuration: java.jdt.ls.serverPath

Added to the Startup configuration section (order: 35). Three behaviours:

Value	Effect
(empty, default)	Bundled Eclipse JDT LS — no change
Directory path	Use that directory as the server home (supersedes JDT_LS_PATH)
Executable file path	Launch the binary directly via stdio transport (e.g. rust-jdt-ls)

Changing the setting triggers the existing "please reload" prompt.

Server startup changes (src/javaServerStarter.ts)

• prepareExecutable() — checks java.jdt.ls.serverPath first; delegates to prepareCustomExecutable() when it resolves to a file.
• New exported prepareCustomExecutable(serverPath, workspacePath, isSyntaxServer) — minimal Executable over stdio; passes --syntax-server in LightWeight/Hybrid mode and workspace via --data.
• prepareParams() — honours the directory variant when resolving the Eclipse server home.

// rust-jdt-ls binary launched as:
// /path/to/rust-jdt-ls [--syntax-server] --data /path/to/workspace
export function prepareCustomExecutable(serverPath, workspacePath, isSyntaxServer): Executable

E2E test runner for rust-jdt-ls (test/runtest-rust.ts)

Reuses the existing lightweight-mode-suite and standard-mode-suite suites unchanged. Configures java.jdt.ls.serverPath via workspace settings.json before each suite run. Binary path is supplied via the RUST_JDT_LS_PATH environment variable; the runner exits cleanly (code 0) when the variable is unset or the binary is absent so CI without rust-jdt-ls is unaffected.

RUST_JDT_LS_PATH=/path/to/rust-jdt-ls npm run test:rust

Original prompt

适配 git@github.com:ifrankwang/rust-jdt-ls.git 项目，确保：1. 可以通过配置切换实际使用的 lsp；2. 以 rust-jdt-ls 作为 lsp 后端，确保其拥有完整的 e2e 功能测试覆盖（如果可以尽量复用现有 e2e 测试），包括 LightWeight 模式和 Standard 模式（无需确保每个测试都能通过，但需要通过使用 eclipse.jdt.ls 作为 lsp 后端来确保 e2e 测试本身没有问题）

Created from VS Code.

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