test(core): add ReadResource response shaping assertions

[yashmittal02]

Description

Adds explicit tests for ReadResource response shaping based on content.type.

The tests verify that:

• text resources return their data using the text field.
• binary resources return Base64-encoded data using the blob field.
• json resources are stringified and returned using the text field.
• The resource mimeType is preserved in each response.
• Valid JSON payloads are handled by the JSON branch without falling through to the default case.

Closes #18

Type of Change

• feat: New feature
• fix: Bug fix
• docs: Documentation update
• refactor: Internal code change
• test: Test-only changes
• chore: Build, CI, or tooling changes

Testing

Ran the targeted core test suite:

npx jest src/core/__tests__/server.extended.test.ts

Result:

• 1 test suite passed
• 5 tests passed
• 0 tests failed

The tests cover text, binary, and json resource response shapes.

Checklist

• My changes are focused on the issue described.
• I have added/updated tests for the changed behavior.
• The relevant test suite passes locally.
• I have followed the project's coding and contribution guidelines.
• No unrelated files are included in this PR.