Add CompatibilitySuppressions.xml for MRTR (SEP-1686) removal

This PR replaces the experimental MRTR (SEP-1686) task APIs shipped in
ModelContextProtocol.Core 1.3.0 with the SEP-2663 tasks extension. NuGet
Package Validation (EnablePackageValidation=true,
PackageValidationBaselineVersion=1.3.0) flags the resulting type and member
removals as breaking changes (CP0001/CP0002/CP0011) and fails 'dotnet pack'
on Release.

Follow the established repo pattern (compare PR #1368 "Add client completion
notification and details", which introduced a 60-line suppressions file in
the same PR that removed McpClient.Completion; that file was deleted in PR
#1621 "Bump version to 2.0.0-preview.1" once the new baseline already
excluded the removed APIs).

Generated via:
  dotnet pack -c Release -p:ApiCompatGenerateSuppressionFile=true

The 320 entries (80 per TFM x 4 TFMs: net8.0, net9.0, net10.0, netstandard2.0)
are entirely MRTR types and members that were marked
[Experimental("MCPEXP001")] in 1.3.0 and are being replaced. A future PR that
bumps PackageValidationBaselineVersion past 2.0.0-preview.1 (once published)
should delete this file in the same way PR #1621 deleted its predecessor.

Verified: 'dotnet pack -c Release' completes with 0 warnings / 0 errors.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: PranavSenthilnathan