Handle JDTLS internal errors in proxy

[imedfan]

JDTLS can return JSON-RPC Internal error responses for user-facing editor requests. In practice that can make completions, code actions, and navigation feel broken even when the language server process is still alive. It can also return signatureHelp.activeParameter = -1, which is invalid for clients expecting an unsigned integer.

This change makes java-lsp-proxy more defensive:

• Tracks editor requests that need URI rewriting or safe fallback handling.
• Converts JDTLS internal errors into safe LSP results for the affected request type.
• Returns the original completion item for failed completionItem/resolve requests.
• Returns [] for failed codeAction, codeLens, and documentHighlight requests.
• Returns null for failed definition, hover, and signature help requests.
• Removes negative activeParameter from successful signature help responses.
• Adds proxy unit tests for internal-error detection, fallback responses, and signature-help sanitizing.

This is intended to improve resilience around failures similar to #226, #68, and #62 without changing managed JDTLS settings or project import defaults.

Validation:

• cargo fmt --all
• cargo check --workspace
• cargo test --workspace
• cargo build --release --target wasm32-wasip1
• cargo build --release -p java-lsp-proxy --target aarch64-apple-darwin

[cla-bot]

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[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @imedfan on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[imedfan]

@cla-bot check

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @imedfan on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[imedfan]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!