ENG-3903: add error handling for missing target uri

[pb-sobrien]

Add validation for target_link_uri in LTI resource messages. Twice in the last week, two orgs have messed up this param (target_link_uri) not being in a launch and thrown a 500. This will catch the error nicely. :)

Purpose

This PR adds validation for the required target_link_uri claim in LTI Resource Link Requests, improving message security and conformance to the LTI 1.3 specification.

Importance

The target_link_uri is a required claim in LTI 1.3 resource link launches that specifies the intended destination URI for the launch. Validating this field:

• Prevents processing incomplete/malformed LTI messages
• Ensures proper implementation of security measures according to the specification
• Protects applications from launch requests that might attempt to bypass launch target validation

Implementation

• Added validation in ResourceMessageValidator::validate() that checks for the presence of the target_link_uri claim
• Added corresponding test case in ResourceMessageValidatorTest to verify the validation behavior
• Uses existing exception handling patterns consistent with other validations

Standards Reference

This change ensures conformance with the IMS Global LTI 1.3 Core Specification which requires the https://purl.imsglobal.org/spec/lti/claim/target_link_uri claim for all resource link
launch messages.

Testing

• I have added automated tests for my changes
• I ran composer test before opening this PR
• I ran composer lint-fix before opening this PR

[pb-sobrien]

tests passing locally

[dbhynds]

@pb-sobrien Can you checkout a branch on our platform, point the LTI Lib to this branch in the composer.json, and make sure tests pass there?

[dbhynds]

Code looks good. Just need to know it passes in our app