Don't mutate the caller's schema in compress_schema - #4492

Closes #4493.

Summary

compress_schema() mutates the caller's schema in place, directly contradicting its own documented contract. _single_pass_optimize (which compress_schema calls) states:

Immutable design prevents shared reference bugs
schema: JSON schema dict to optimize (not modified)
Returns: A new optimized schema dict

But its pruning phases .pop() title, additionalProperties, and unused $defs off the passed-in dict:

>>> from fastmcp.utilities.json_schema import compress_schema
>>> schema = {"type": "object", "title": "S", "additionalProperties": False,
...           "properties": {"a": {"type": "string", "title": "A"}},
...           "$defs": {"Unused": {"type": "string"}}}
>>> compress_schema(schema, prune_titles=True, prune_additional_properties=True)
{'type': 'object', 'properties': {'a': {'type': 'string'}}}
>>> schema  # the caller's object was stripped in place
{'type': 'object', 'properties': {'a': {'type': 'string'}}}

This bites at real call sites that pass a live object — e.g. client/sampling/handlers/google_genai.py calls compress_schema(tool.input_schema, ...) when converting a tool for Gemini, which permanently strips title/additionalProperties/$defs off the shared Tool object — exactly the "shared reference bug" the docstring says it prevents.

Fix

Deep-copy the schema before the in-place pruning phases, matching the sibling _prune_param which already does schema = copy.deepcopy(schema). One line; copy is already imported. Because the copy happens at the single entry point (_single_pass_optimize), it covers every compress_schema caller.

Test

Adds TestCompressSchema::test_does_not_mutate_input (mirroring the existing _prune_param non-mutation test): a schema with a title, additionalProperties, nested titles and an unused $def is left untouched after compress_schema(..., prune_titles=True, prune_additional_properties=True), and the returned copy is confirmed to actually be optimized (so it isn't a no-op). It fails on the current code and passes with the fix; the full test_json_schema.py suite passes and ruff check + format are clean.