Chunked upload: 16 GB segment count and APPEND pacing

[mintlify]

Addresses the docs gap after the 16 GB Premium media limit (forum: https://devcommunity.x.com/t/276668).

Changes to x-api/media/quickstart/media-upload-chunked.mdx:

• Note that 16 GB uploads need about 2,000 (8 MB) to 3,300 (5 MB) APPENDs, and that segment_index is not capped at 999.
• New "Pace APPEND requests for large files" section: the 1,875/15 min user limit, reading x-rate-limit-* headers, waiting for reset on 429, and resuming from the failed segment instead of restarting.

Not in this PR:

• AppendMediaUploadRequest.segment_index maximum: 999 in openapi.json. That file is overwritten from https://api.x.com/2/openapi.json by update-docs.yml four times a day, so the fix has to land in the upstream spec. The XChat append schema also has maximum: 999.
• Any change to the published APPEND rate limits. The API team needs to decide this.

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Generated by Mintlify Agent.

Requested by: tcaldwell@x.ai via Slack
Mintlify session: slack_1790353489.651319_C09MQDT6BM4

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