fix(client): paginate large dataframe span exports

[Hasnaathussain]

Resolves #11873

The common unfiltered get_spans_dataframe() path now uses the existing cursor-paginated spans endpoint and combines all pages into the same dataframe shape. This avoids sending a large export through the legacy single-request query endpoint.

Queries that use the SpanQuery DSL or root_spans_only keep the legacy endpoint so their existing filtering semantics are unchanged. Both sync and async clients are covered.

Verification:

• 41 spans resource tests passed
• Ruff checks and formatting passed
• Mypy passed for the spans resource
• DCO sign-off included

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[Hasnaathussain]

I have read the CLA Document and I hereby sign the CLA

[Hasnaathussain]

recheck

[Hasnaathussain]

The visible checks for #15584 (CLA, external-issue labeling, and the four Read the Docs builds) are green. The Python, TypeScript, Playwright, Semantic PR, Claude review, publish, zizmor, and phoenix-sqlean workflows are held in action_required before any job starts on this forked PR. Could a maintainer approve the queued workflows when convenient? Local focused tests, Ruff, mypy, and diff checks pass; no code-caused failure is known.