Add 2025-11-25 as supported version

[Copilot]

Updates the default MCP protocol version to 2025-11-25, following the pattern established in #529.

Changes

• Core: Set LatestProtocolVersion to "2025-11-25" and added "2025-06-18" to SupportedProtocolVersions array
• Documentation: Updated spec links from 2025-06-18 to 2025-11-25 in XML doc comments and markdown files
• Samples/Tests: Updated MCP-Protocol-Version headers and protocolVersion values in .http files
• Test assertions: Updated expected default version to 2025-11-25; test data now uses 2025-06-18 for backward compatibility checks

internal const string LatestProtocolVersion = "2025-11-25";

internal static readonly string[] SupportedProtocolVersions =
[
    "2024-11-05",
    "2025-03-26",
    "2025-06-18",
    LatestProtocolVersion,
];

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add 2025-11-25 as supported version</issue_title>
<issue_description>Update the default protocol version to 2025-11-25.

For reference, this was done in #529 for the 2025-06-18 spec version.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add 2025-11-25 as supported version #1129

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[stephentoub]

@mikekistler, we don't yet support MCP tasks... do you want to merge this anyway?

[mikekistler]

Looks good. 👍

[mikekistler]

@stephentoub I don't think we need to wait for Tasks support as it is experimental, but the support for SEP-1699 is still not merged and I think we should wait for that. We could also wait for the Tasks support if it is just a matter of a few days, but otherwise I think we let it come later.

[jeffhandley]

Marking as NO MERGE PR should not be merged until the label is removed until Streamable HTTP resumability + redelivery + SSE polling via server-side disconnect (#1077) is merged to resolve SEP-1699: Support SSE polling via server-side disconnect (#1020).