[php] Set $multipart to true if content-type is multipart/form-data (#5536)

The PHP client does not correctly identify multipart/form-data requests without files. See https://github.com/OpenAPITools/openapi-generator/issues/5536#issuecomment-2182344467 for details. This PR fixes this bug.

[x] Read the contribution guidelines.
[x] Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
[x] Run the following to build the project and update samples:
```
./mvnw clean package 
./bin/generate-samples.sh ./bin/configs/*.yaml
./bin/utils/export_docs_generators.sh
```
(For Windows users, please run the script in Git BASH) Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
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OpenAPITools / openapi-generator