Improve launch of python-based LSP server within IDEs

[asnare]

Changes

This PR tweaks the way we launch python-based LSP launchers, mainly to help with developer workflows in the latest IntelliJ release. The issue here is that when running individual tests via the IDE, as of the latest release the (hatch-created) virtual environment is not activated: this causes many tests to run the stub LSP server against the system python, or fail because the system only provides python3 (and not python).

Relevant implementation details

The key change here is that if the LSP server is python-based but there's no dedicated .venv for it, the LSP server is now always launched with the current python interpreter.

As a related minor improvement, we also raise an error earlier if the executable we're about to launch can't be found.

Tests

• manually tested
• existing unit tests
• existing integration tests

[codecov]

Codecov Report

❌ Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.52%. Comparing base (5a297d2) to head (25b3d59).

Files with missing lines	Patch %	Lines
...ricks/labs/lakebridge/transpiler/lsp/lsp_engine.py	37.50%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2184      +/-   ##
==========================================
- Coverage   63.56%   63.52%   -0.04%     
==========================================
  Files         100      100              
  Lines        8503     8508       +5     
  Branches      885      887       +2     
==========================================
  Hits         5405     5405              
- Misses       2931     2934       +3     
- Partials      167      169       +2     

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[github-actions]

✅ 51/51 passed, 10 flaky, 3m56s total

Flaky tests:

• 🤪 test_validate_successful_schema_check (203ms)
• 🤪 test_validate_invalid_schema_check (1ms)
• 🤪 test_validate_non_empty_tables (11ms)
• 🤪 test_validate_invalid_source_tech (315ms)
• 🤪 test_validate_table_not_found (1ms)
• 🤪 test_transpiles_informatica_to_sparksql (21.116s)
• 🤪 test_transpile_teradata_sql_non_interactive[False] (22.696s)
• 🤪 test_transpile_teradata_sql (24.66s)
• 🤪 test_transpile_teradata_sql_non_interactive[True] (5.985s)
• 🤪 test_transpiles_informatica_to_sparksql_non_interactive[False] (12.85s)

_{Running from acceptance #3150}