fix(healthcare): enforce json responseType and fix IAM payloads in FHIR samples

[angelcaamal]

Description

Fixes #

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

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[gemini-code-assist]

Code Review

This pull request updates several FHIR-related scripts to include "responseType: 'json'" in API requests and implements robust error handling using try-catch blocks. Notably, "exportFhirResources.js" now uses a polling mechanism to check operation status instead of a fixed delay, and "setFhirStoreIamPolicy.js" was corrected to pass "members" as an array. Review feedback suggests explicitly setting the "Content-Type" header in "searchFhirResourcesPost.js" and adding a null check for the "entry" field in FHIR search results to prevent runtime errors.

[iennae]

it's helpful if you put a replaces issue # (and a line for each one)