Fix spelling errors and add defensive coding for authority URLs

[Copilot]

Description

Code quality improvements in SMART configuration handling:

• Fixed test method naming: Specifired → Specified, ShouldContains → ShouldContain
• Added defensive trimming of trailing slashes in GetAuthority() to prevent malformed URLs like https://authority.com//authorize

private string GetAuthority()
{
    var authority = !string.IsNullOrEmpty(_smartIdentityProviderConfiguration.Authority) ?
        _smartIdentityProviderConfiguration.Authority : _securityConfiguration.Authentication.Authority;
    return authority?.TrimEnd('/');
}

Related issues

Addresses [issue #175966].

User Story 175966: Complete .well-known/smart-configuration for a 3rd-party IDP in Paas Fhir service for SMART v2.

Testing

Existing unit tests validate behavior. Updated test expectations to align with trailing slash normalization.

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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