[rust] fix: exploded deepObject free-form query parameters do not compile

[wiebren]

A query parameter with style: deepObject, explode: true whose schema is a bare free-form
object (type: object, no additionalProperties) generates iteration code that cannot
compile: the exploded deepObject branch walks every map-flagged parameter with
.len()/.iter(), which exist on the HashMap a declared-additionalProperties schema
produces but not on the serde_json::Value a bare free-form schema produces.

This is the second of the two follow-ups promised in #24866's body ("the exploded
deepObject free-form branch iterates a serde_json::Value"); the first became #24896.

The fix

The deepObject {{#isMap}} branch splits on isContainer — exactly the flag that separates
the two typings (#24866 uses the same distinction). The HashMap shape keeps its direct
iteration; the serde_json::Value shape walks the value through as_object():

if let Some(object) = param_value.as_object() {
    let mut query_params = Vec::with_capacity(object.len());
    for (key, value) in object.iter() {
        query_params.push((key.to_string(), serde_json::to_string(value)?));
    }
    req_builder = req_builder.query(&query_params);
}

A non-object Value sends nothing, matching the parameter's declared object shape. Four
template sites: the nullable-required and optional paths in reqwest and reqwest-trait
(hyper has no deepObject-explode branch).

Tests

RustClientCodegenTest#testDeepObjectFreeFormQueryParamCompiles generates the new
3_0/rust/deep-object-free-form-query-param.yaml fixture (one deepObject parameter of each
shape) for both libraries and asserts the as_object() walk next to the direct map
iteration. Fails without the template change (verified by stashing it).

Verified by cargo build of clients generated from the fixture for both libraries: clean
with this change. Note the bare free-form parameter's type additionally needs #24866
(models::serde_json::Value today); the two changes touch different lines and compose —
the builds were verified with both applied.

PR checklist

• Read the contribution guidelines.
• Built the project and updated samples (./bin/generate-samples.sh ./bin/configs/rust-*):
  zero diffs - no sample spec has an exploded deepObject free-form parameter.
• Technical committee (Rust): @frol @farcaller @richardwhiuk @paladinzh @jacob-pro @dsteeley

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Generated with Claude Code

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Summary by cubic

Fixes Rust codegen for exploded deepObject query parameters with bare free-form object schemas, which previously generated uncompilable iteration code. Both reqwest and reqwest-trait now route exploded deepObject maps through parse_deep_object, which also changes typed maps to emit the name[key]=value wire format instead of json-quoted pairs.

Bug Fixes

• Applies parse_deep_object to all four exploded deepObject map sites across both templates.
• Adds a test fixture and generator test covering optional typed maps, optional free-form objects, and required-nullable maps in both libraries.

^{Written for commit 3590759. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 4 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[wiebren]

cubic's P1 was right, and it applied to the pre-existing HashMap branch as much as to the new one: neither emitted deepObject form. Rather than patch the free-form copy, both shapes now route through crate::apis::parse_deep_object - the same call the non-explode branch already uses - so an exploded style: deepObject map reaches the wire as name[key]=value with plain string values, for the typed-map and serde_json::Value shapes alike, in one code path. Note this changes the wire output of the previously-compiling HashMap case from unprefixed json-quoted pairs to actual deepObject form; that output never matched the declared style, so this is the same fix, not a side effect.

The two test remarks are addressed too: the fixture gains a required-nullable deepObject parameter so every patched site is exercised, and the generated tree is registered for cleanup (files.forEach(File::deleteOnExit)).

Re-verified: cargo build clean for both libraries from the fixture (with #24866's typing fix applied on top for the bare free-form parameter's type), zero sample diffs.