RFC 9728 resource is used instead of base url when possible

[fkonecny]

Motivation and Context

Library initiates OAuth2 workflow with predefined resource as the base url to abide RFC 8707. This violates a possibility where MCP belongs under umbrella of exposed functionalities. RFC 8707 permits it in its second paragraph.

There was already implemented resource discovery from OAuth2 resource metadata document defined by RFC 9728 in PR 937 which can access the exposed resource.

Use resource defined by OAuth2 resource metadata document so the creator of MCP and its provider can control what resource are tokens requested for.

How Has This Been Tested?

2 tests were added:

• OAuth2 resource metadata document is missing,
• OAuth2 resource metadata document declares resources.

PR 937 already handles situations where document has missing resource.

Breaking Changes

Breaking change: changes default behavior where base url is utilized always for resource and use defined resource by provider when available.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[alexhancock]

@filip-konecny-tfs Can you take a look at the red checks? Once resolved we can move forward

[fkonecny]

@alexhancock I did squash the commits and shortened the message so I can pass the lint message checks. Which erased the history of checks (sorry).

The public API check is confusing me as it refers things which I did not initiate, at least I think.

The semver check is confusing me, should I be the one bumping it? I can if you wish to do so.

P.S. @filip-konecny-tfs is my work account which I should not use for this type of activities :-).

[DaleSeo]

Here, expected is base_url, and actual is the provider-declared resource. This means it accepts any declared path where the base_url path starts with that path. For example, if base_url is https://mcp.example.com/mcp-tools and the metadata declares https://mcp.example.com/mcp, then "/mcp-tools".starts_with("/mcp") is true.

[DaleSeo]

nit

Suggested change

	let Some(resource_url) = Url::parse(resource).ok() else {
	return Err(AuthError::MetadataError(
	"Protected resource metadata resource field is not a valid URL".to_string(),
	));
	};
	let Ok(resource_url) = Url::parse(resource) else {
	return Err(AuthError::MetadataError(
	"Protected resource metadata resource field is not a valid URL".to_string(),
	));
	};