fix: parse Accept header media type to allow parameters like charset=utf-8

[MadhukarJukanti]

Fixes #3876

Description

---Problem

The $export endpoint validates the Accept header using exact string comparison against application/fhir+json.
When .NET's HttpClient makes the request, it automatically appends ; charset=utf-8, producing
application/fhir+json; charset=utf-8 — which the exact match rejects with a 400 error.

Fix

Replaced string.Equals with MediaTypeHeaderValue.TryParse so only the media type portion is compared, ignoring
any additional parameters such as charset.

Tests Added

Three new test cases in ValidateExportRequestFilterAttributeTests:

• application/fhir+json; charset=utf-8
• application/fhir+json;charset=utf-8 (no space after semicolon)
• application/fhir+json; charset=UTF-8 (uppercase charset)

All three now pass without throwing RequestNotValidException.

Related issues

Addresses [issue #].

Testing

Describe how this change was tested.

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• When changing or adding behavior, if your code modifies the system design or changes design assumptions, please create and include an ADR.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Patch|Skip|Feature|Breaking (reason)

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