feat: add Gleam language support via the Gleam compiler's bundled gleam lsp

[weiconghe]

What does this PR do?

Adds Gleam language support to solidlsp, using the language server bundled with the Gleam compiler (gleam lsp). Closes #1334 (good first issue).

Implementation

• New GleamLanguageServer (src/solidlsp/language_servers/gleam_language_server.py) using the LanguageServerDependencyProviderSinglePath pattern (same as qmlls): discovers gleam on PATH, launches [gleam, "lsp"].
• LanguageServerId.GLEAM registered in ls_config.py (enum + *.gleam matcher + factory), in the main (non-experimental) section alongside QML.
• Readiness: gleam lsp emits $/progress (begin/end) for its initial dependency-resolution phase on first start. The server tracks active progress tokens and waits for all phases to end before declaring ready (bounded: 10 s for the first begin, 180 s for all to drain). Once dependencies are resolved, symbol / reference / hover / diagnostics queries are served synchronously — no per-request compile-wait or retry is needed (verified directly against gleam 1.17).
• Dependencies are pre-fetched via gleam deps download in _start_server so the LSP's own download phase is short and there is no first-request race on slow CI runners.
• _supports_pull_diagnostics overridden to return False: gleam lsp does not implement textDocument/diagnostic (no diagnosticProvider capability is advertised). Sending a pull request hangs (the server neither responds nor errors), so we opt out and rely on textDocument/publishDiagnostics (push), which the base class stores via _observe_server_notification.

Why not build on #1357?

This PR supersedes two prior attempts that went stale:

• feat: add Gleam language support #1352 (jakeefr) — closed by a maintainer ("No reaction, closing")
• Add Gleam language server support #1357 (Koushik-Salammagari) — open but abandoned (last activity 2026-04-21, 3+ months stale; MischaPanch's "you are still on this, right?" went unanswered)

Diagnosing why #1357's CI was flaky surfaced two root causes, both fixed here:

1. Fixture import syntax was wrong. calculator.gleam used import test_repo/utils, which Gleam 1.x rejects with Unknown module. The correct form is import utils (in-project module paths are relative to src/, without the project-name prefix). This is why "reference-finding tests fail" was observed on Add Gleam language server support #1357 — the cross-file reference target was never resolvable, so the cross-file references test could not pass regardless of timing. (Credit to Koushik-Salammagari for the original fixture structure and test coverage; the import line is the fix.)

2. textDocument/diagnostic (pull) hung the diagnostics test. Gleam LSP advertises no diagnosticProvider; the base class's default _supports_pull_diagnostics → True sent a pull request the server never answered, timing the test out (240 s). Overriding to False routes through publishDiagnostics (push) instead.

#1357 was also built against a now-outdated main: it used ProcessLaunchInfo and the old _get_initialize_params(repository_absolute_path) signature, both superseded by the DependencyProvider + _create_base_initialize_params refactors. This PR is built on current main with the modern patterns (the same ones requested in the QML review of #1635).

Notes

• The Gleam compiler is a self-contained Rust binary — neither Erlang/OTP nor a separate LSP package is required for symbol / reference / hover / diagnostics queries. (Full project compilation to BEAM bytecode does require Erlang, but the LSP's analysis path does not.)
• gleam deps download runs at server start and only needs network access to the Hex package mirror; no Erlang runtime.

How did you verify your code works?

• Local (Windows 11, gleam 1.17.0 standalone binary): all 6 tests pass in ~16 s:
  • test_ls_is_running
  • test_document_symbols — asserts add / subtract / multiply / Calculator / demo are present
  • test_find_symbol — request_full_symbol_tree (backing serena's find_symbol tool) sees symbols from both calculator.gleam and utils.gleam
  • test_find_references_within_file — add definition → its call site in demo (same file)
  • test_find_references_across_files — format_output defined in utils.gleam → its call sites in calculator.gleam
  • test_file_diagnostics — undefined_symbol in diagnostics_sample.gleam is flagged
• CI (Linux ubuntu-latest, gleam 1.17.0 x86_64-musl): gleam is installed on the other-langs Linux batch (mirrors the qmlls setup); the gleam marker is added to MARKERS_OTHER_LANGS. Tests skip on Windows/macOS CI and off-CI without the binary via the central conftest guard.
• ruff check, ruff format, and ty check are all clean.

Checklist

• This PR follows the guidelines in CONTRIBUTING.md regarding the scope of PRs.
• For changes that add features or fix problems, I have added an entry to CHANGELOG.md.