api: fix --all returning no output on single-page results

[jordanenglish]

Description

tfctl api --all returns no output when the result fits in a single page. In
paginateResponse, the initial response body is read in full to look for a
next pagination link. On the single-page path (no next link, or the payload
is not a paginated collection) the function returned the response without
restoring that drained body, so rendering saw an empty body. In practice this
surfaced as a not a JSON:API envelope error rather than the expected result.

This restores the buffered body before returning on that path, mirroring what
the multi-page path already does when it re-wraps the merged body.

Example Output

Before (against a resource whose result fits on one page):

$ tfctl api /organizations/my-org/workspaces --all
Error: not a JSON:API envelope

After:

$ tfctl api /organizations/my-org/workspaces --all
# renders the workspaces as usual

PR Checklist

• Run npx changie new or install changie to prepare a new changelog entry for the next set of release notes.
  • Added .changes/unreleased/BUG FIXES-*.yaml (kind: BUG FIXES).
• Ensure any command changes are sensitive to these global flags:
  • --json — Force machine readable output to stdout. Does not apply to stderr.
  • --markdown — Force markdown output to stdout. Does not apply to stderr.
  • --dry-run — Don't make any actual writes or other mutations. Describe what would have changed to stderr.
  • --quiet — Don't render output to stdout.
  • No new flags or output modes. The fix restores the response body so all existing output paths (--json/--markdown/--quiet) render as they would for a non---all request. --dry-run is not applicable to api GET pagination.
• Get the logging interface from the context and add debug logging for interesting conditions and nonfatal situations.
  • No new nonfatal conditions are introduced.
• Run make gen/screenshot if the root command output changes.
  • Not required: root command output is unchanged.
• Add the Autocomplete field to positional arguments and flags to assist shell autocomplete.
  • N/A: no new arguments or flags.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

  • Reverting the PR fully removes the change; no additional revert steps or data migration.

• If applicable, I've documented the impact of any changes to security controls.

  • N/A: no security-control changes.

[brandonc]

Nice and neat fix