ci: preview Python 3.14 and 3.15 support

[AzulGarza]

Summary

• Adds CI job test-future-python on 3.14 and 3.15 (same pytest suite as test, continue-on-error: true) so we can track dependency wheels and model tests without blocking merges.
• Documents preview support and model availability in docs/contributing.md and AGENTS.md.
• Adds PyPI classifier Python 3.14 and includes Timestral in package import smoke tests.

Context

requires-python is already >=3.10. Several optional deps stay capped below 3.14 (granite-tsfm, tfc-t0, uni2ts) or 3.13 (TabPFN, Sundial); FlowState / PatchTST-FM / T0 remain 3.11–3.13. Chronos, TimesFM, Timestral, TiRex, etc. are the main candidates on 3.14+ once torch/transformers wheels are stable.

Test plan

• Preview job results on 3.14 / 3.15 (may fail until upstream wheels land)
• Required matrix 3.10–3.13 unchanged

Follow-up

When preview CI is consistently green, promote 3.14 (then 3.15) into the required test matrix and drop continue-on-error.

Made with Cursor

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[codspeed]

Merging this PR will improve performance by 10.57%

ℹ️ 13 benchmarks have no flame graph

The flamegraph could not be generated because we failed to resolve the symbols in the profiling trace. This happens when we fail to harvest information about language frames.

• test_maybe_infer_freq
• test_timeseries_dataset_from_panel
• test_quantile_converter_level_to_quantiles
• test_seasonal_naive_forecast
• test_process_panel_from_df_string_ds
• test_timeseries_dataset_from_df[small]
• test_assign_quantile_forecasts
• test_timeseries_dataset_from_df_with_panel
• test_foundation_forecast_multi_model[5-models]
• test_process_panel_from_df[small]
• …and 3 more

⚡ 1 improved benchmark
✅ 16 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	test_model_forecast[moirai-1.1]	1.5 s	1.4 s	+10.57%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ci/python-3.14-3.15 (a1fa59e) with main (5d833ee)}