Wiebren fix/rust deep object free form

[wing328]

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Summary by cubic

Fixes Rust template generation for exploded deepObject map and free-form object query parameters, which previously emitted unprefixed, JSON-quoted query pairs and produced non-compiling code for free-form objects.

• Both typed map and free-form serde_json::Value parameters now route through crate::apis::parse_deep_object, matching the non-explode branch and producing the name[key]=value wire format.
• Adds a test fixture and unit test covering optional and required-nullable deep-object parameters, with samples regenerated across all Rust libraries.

^{Written for commit ce26f36. Summary will update on new commits.}