[dart] fix: explode object query parameters

[wiebren]

A query parameter whose schema is an object and whose style/explode are left at their
defaults — style: form, explode: true — must go on the wire as one parameter per entry,
keyed by the property name alone. The dart client stringified the whole map with
Map.toString() into a single parameter instead, while csharp, java and php got it
right.

Given:

parameters:
  - in: query
    name: filter
    schema:
      type: object

called with {"tld": "com", "createdDate:gte": "2023-01-01"}:

generator	on the wire
expected	tld=com&createdDate%3Agte=2023-01-01
dart	filter=%7Btld%3A+com%2C+createdDate%3Agte%3A+2023-01-01%7D — dart's Map.toString(), percent encoded
csharp, java, php	correct

Sibling of #24797 (go), #24802 (python) and #24803 (typescript-fetch), one PR per language
per the maintainer's request on #24797. This PR is independent of the other three — no
shared main/ code. It adds the same test fixture at the same path with identical content,
so whichever lands first, the others rebase cleanly.

The cause

dart2/api.mustache — every query parameter was handed whole to _queryParams, which
receives only a collection format (empty for a map) and falls through to
parameterToString(value), i.e. value.toString(). No style or explode reaches the
runtime.

The fix mirrors the accepted python change in #24802: the generated api iterates an exploded
map entry by entry at the call site —

(filter as Map).forEach((entryKey, entryValue) => queryParams.addAll(_queryParams('', entryKey.toString(), entryValue)));

— and _queryParams is left alone, since it is shared by every call site and is still the
right fallback for a parameter that is not an exploded map. A free-form object is typed
Object in dart and needs the cast; a declared map does not get one, so the analyzer stays
clean (dart analyze on the generated fixture client: no issues). deepObject and
explode: false parameters keep the exact line they produced before.

Verified on the wire, not just asserted

The client was generated from the new fixture and pointed at a server that echoes its own raw
request target back:

parameter	before	after
filter (object, defaults)	filter=%7Btld%3A+com%2C+createdDate%3Agte%3A+2023-01-01%7D	tld=com&createdDate%3Agte=2023-01-01
typedFilter (map, defaults)	same Map.toString() shape	tld=com&createdDate%3Agte=2023-01-01
deepFilter (style: deepObject)	deepFilter=%7Btld%3A+com%2C…%7D	unchanged, byte for byte
flatFilter (explode: false)	flatFilter=%7Btld%3A+com%2C…%7D	unchanged, byte for byte

Known gaps, called out deliberately

Declared object models. A $refed object model as a query parameter is isModel, not
isMap, so it still goes on the wire whole. Pre-existing, and the same gap the sibling PRs
document — iterating a model would put the dart property names on the wire rather than the
wire names. Left for a follow-up.

deepObject and explode: false keep their previous single-Map.toString() parameter.
Both are wrong per the spec (deepFilter[tld]=com and flatFilter=tld,com,… respectively),
both were wrong before this change, and both go on the wire byte for byte as they did
before — same scoping as the sibling PRs.

A non-map value in a free-form object parameter now throws a TypeError at the cast
instead of going on the wire as its toString(). This matches python, where a non-dict
raises at .items().

Tests

modules/openapi-generator/src/test/resources/3_0/exploded-object-query-param.yaml covers
the four combinations that decide the wire format:

• DartClientCodegenTest#testExplodedObjectQueryParameter

It fails without the fix (verified by stashing only the template change). All 13 tests in
DartClientCodegenTest pass.

PR checklist

• Read the contribution guidelines.
• Built the project and updated samples (./bin/generate-samples.sh for
  bin/configs/dart-*.yaml; ./bin/utils/export_docs_generators.sh produced no diff).
  1 sample file changed: petstore_client_lib_fake/lib/api/fake_api.dart, from the
  petstore fixture's language parameter — a declared map with the default style, the
  same parameter that changed in the python sibling. The dart-dio samples are untouched,
  since that generator has its own templates.
• Technical committee: @yissachar @joernahrens @swipesight @jaumard

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Generated with Claude Code

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Summary by cubic

Fixes the Dart client so object query parameters with the default style: form and explode: true go on the wire as one parameter per entry, keyed by property name, instead of as a single Map.toString()-stringified parameter.

Behavior

• Free-form objects and declared maps are now iterated entry by entry at the generated call site.
• deepObject and explode: false parameters keep their previous wire format, byte for byte.
• A non-map value in a free-form object now throws a TypeError at the cast instead of being stringified.

Known gaps

• $refed object models as query parameters are still sent whole; pre-existing behavior left for a follow-up.
• deepObject and explode: false output remains spec-incorrect but unchanged from before.

^{Written for commit bbcc510. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}