Ensure signature help label contains parameter substrings per LSP spec

[SoulSniper-V2]

Problem

When signature help is requested for callable objects whose docstring header does not explicitly list the parameter signature (for example, callable class instances or C-extensions/ufuncs), pylsp could return a truncated label like MyCallable() or ufunc() while parameters lists parameter names like ['a', 'b'] or ['args', 'kwargs'].

According to the LSP 3.17 specification:

Note: a label of type string should be a substring of its containing signature label. Its intended use case is to highlight the parameter label part in the SignatureInformation.label.

Because parameter labels were not substrings of the signature label, strict LSP clients (such as YouCompleteMe) report a protocol error / invalid signature message.

Reported in #678.

Root cause

In pylsp/plugins/signature.py:

1. function_sig was parsed by splitting s.docstring() on \n\n and taking the first line/paragraph. For callable instances or objects whose docstring begins with a class or type header without parameter names (e.g. ufunc()), this produced a header without parameters.
2. s.params was populated with parameter objects whose names (p.name) were not contained in the truncated label.

Fix

1. Use Jedi's s.to_string() to obtain the canonical signature string (e.g. MyCallable(a, b=1)), falling back to docstring parsing if to_string() is empty or unavailable.
2. If s.params is present and any parameter name is not a substring of function_sig, construct the label as f"{name}({param_list})" so that all parameter names are guaranteed to be substrings of the signature label.
3. As an additional safeguard, ensure parameter entries in sig["parameters"] are only included if their label is a substring of the containing signature label per the LSP specification.

Test

Added unit tests in test/plugins/test_signature.py:

• test_callable_instance_signature: Verifies that signature help for a callable class instance includes its parameters in the signature label and that all parameter labels are substrings of the signature label.
• test_signature_fallback_when_label_missing_params: Verifies fallback formatting when a signature label would otherwise omit parameter names.

Fixes #678

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