feat: created api to get course modes in instructor app

[AhtishamShahid]

Description

Issue : #37465

This PR added API endpoint to get course modes for instructor dashboard .

/api/instructor/v1/courses/{course_key}/modes

HTTP 200 OK
Allow: GET, HEAD, OPTIONS
Content-Type: application/json
Vary: Accept

{
    "modes": [
        {
            "mode_slug": "audit",
            "mode_display_name": "Audit Track",
            "min_price": 0,
            "currency": "USD",
            "expiration_datetime": null,
            "description": "Access the course materials for free, but without a certificate.",
            "sku": null,
            "android_sku": null,
            "ios_sku": null,
            "bulk_sku": null
        },
        {
            "mode_slug": "verified",
            "mode_display_name": "Verified Track",
            "min_price": 49,
            "currency": "USD",
            "expiration_datetime": null,
            "description": "Access all materials, graded assignments, and earn a verified certificate.",
            "sku": null,
            "android_sku": null,
            "ios_sku": null,
            "bulk_sku": null
        }
    ]
}

[jawad-khan]

Typo?

[jawad-khan]

This is a redundant check, if course doesn't exist we will get 404 in permission check. we can safely skip this.

[Copilot]

Pull Request Overview

This PR adds a new API endpoint to retrieve course enrollment modes (e.g., audit, verified) for instructors and staff, along with comprehensive test coverage.

• Adds CourseModeListView API endpoint for fetching enrollment modes
• Creates CourseModeSerializer and CourseModeListSerializer for API responses
• Implements authentication/authorization checks and comprehensive tests

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 10 comments.

File	Description
lms/djangoapps/instructor/views/serializer.py	Adds CourseModeSerializer and CourseModeListSerializer to serialize course mode data
lms/djangoapps/instructor/views/api_urls.py	Registers the new course_modes_list endpoint in the v1 API URL patterns
lms/djangoapps/instructor/views/api.py	Implements CourseModeListView class with GET handler and documentation
lms/djangoapps/instructor/tests/test_api.py	Adds comprehensive test suite covering authentication, permissions, and response validation

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