test: add regression test for issue #2001 (progress related_request_id)

[kaushik701]

Adds a dedicated regression test file for issue #2001.

Context.report_progress() was silently dropping progress notifications in stateless HTTP / SSE transports because send_progress_notification() was called without related_request_id. The fix (already in main via mcpserver/context.py) passes related_request_id=self.request_id, consistent with send_log_message().

This test file covers:

• The primary regression: related_request_id is forwarded on every call
• Edge case: no progress_token -> notification is skipped (no-op)
• Edge case: integer progress_token (e.g. 0) works correctly

Closes #2001

Motivation and Context

How Has This Been Tested?

Breaking Changes

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

[StantonMatt]

I checked this against the current test tree and I think this PR mostly duplicates coverage that is already on main.

The main regression for #2001 is already covered by tests/server/mcpserver/test_server.py::test_report_progress_passes_related_request_id, including the related_request_id assertion. The integer progress_token=0 case is already covered by tests/issues/test_176_progress_token.py::test_progress_token_zero_first_call, also with related_request_id. The no-token no-op behavior is covered at the interaction level by tests/interaction/mcpserver/test_context.py::test_report_progress_without_a_progress_token_sends_nothing.

I verified this branch and the overlapping tests locally on 0d3e9ef:

• uv run --frozen pytest tests/issues/test_2001_progress_related_request_id.py tests/server/mcpserver/test_server.py::test_report_progress_passes_related_request_id tests/issues/test_176_progress_token.py -q -> 5 passed
• uv run --frozen ruff check tests/issues/test_2001_progress_related_request_id.py tests/server/mcpserver/test_server.py tests/issues/test_176_progress_token.py
• uv run --frozen ruff format --check tests/issues/test_2001_progress_related_request_id.py tests/server/mcpserver/test_server.py tests/issues/test_176_progress_token.py
• uv run --frozen pyright tests/issues/test_2001_progress_related_request_id.py

So the tests pass, but I do not see much new coverage from adding this extra issue file as-is. If maintainers want an issue-specific file for #2001, it may be worth trimming it to only a missing case, otherwise this can probably be closed in favor of the existing regression and interaction tests.