[Website] Document the 16.7 response format selection changes

[glen-84]

Summary

• Adds a Behavioral breaking changes entry to the 16.6 to 16.7 migration guide for the media type selection in Fix GraphQL over HTTP conformance gaps in the HTTP pipeline #10392 and Answer a single result as multipart or SSE only when the client names it #10455. The response format follows the Accept header's quality values, a q=0 media type is never chosen, and a single result is written as multipart/mixed or text/event-stream only when the header names that media type or its type/* range.
• A table lists the headers whose response changes, for single results and for @defer and @stream results. The entry also states that a tie between the two envelopes goes to multipart/mixed, and when a 406 has no body.
• Every row was measured against the 16.6.7 package and main, under the Legacy and default transport versions.

[Copilot]

Copilot review overview

🟢 Approval recommended

The migration guidance is complete and no unresolved issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Documents the 16.7 response format negotiation changes and migration impact.

Changes:

• Explains Accept quality values and q=0 handling.
• Documents single and streaming response behavior.
• Clarifies envelope tie-breaking and bodyless 406 responses.

File	Description
website/​content/​docs/​hotchocolate/​migrating/​migrate-from-16-6-to-16-7.md	Adds response format migration guidance and behavior examples.

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