fix(aws-strands): raise Python ceiling to support 3.14

[aviau]

Problem

ag_ui_strands declares requires-python = ">=3.12, <3.14", which blocks
installation on Python 3.14 even though nothing in the package or its
dependency tree actually needs the cap.

Why 3.14 is safe

All direct dependencies already support it:

• strands-agents, ag-ui-protocol, fastapi — no upper bound
• ag-ui-a2ui-toolkit — >=3.10, <3.15

Changes

• pyproject.toml — requires-python = ">=3.12, <3.15"
• examples/pyproject.toml — python = "<3.15,>=3.12"
• examples/README.md — supported range now reads 3.12 – 3.14; drops the
  poetry env use python3.13 workaround that steered around 3.14
• uv.lock — regenerated; adds cp314 wheels. Also picks up
  ag-ui-a2ui-toolkit 0.0.3 → 0.0.4, matching the same incidental lock-drift
  fix noted in fix(aws-strands): lower Python floor to 3.10 #2285 (pyproject already required >=0.0.4, the lock was
  still on 0.0.3)

Verification (CPython 3.14.6)

• uv sync --python 3.14 --all-groups — resolves and installs cleanly
• import ag_ui_strands — all 20 public exports load
• pytest tests/ — 176 passed, 2 skipped (same counts as the 3.10 floor
  verification in fix(aws-strands): lower Python floor to 3.10 #2285)
• poetry install in examples/ on a 3.14 interpreter — resolves and
  installs; import server loads the full app graph (only fails at
  model-construction time on a missing OPENAI_API_KEY, unrelated to
  Python version)
• uv build — wheel reports Requires-Python: <3.15,>=3.12

This is independent of #2282/#2285 (which only touch the floor) — happy to
rebase on top if that lands first.