Scope `plt.style.use` and `sns.set_palette` so they're not global

[DevanshSrajput]

Context
Agent.py:942-943 mutates matplotlib's and seaborn's global
rcParams from inside create_visualizations. That means every
other matplotlib consumer in the same Python process inherits
the "default + husl" look — and re-instantiating the agent
silently reverts the user's style. The fix is to scope the
mutation to the chart-rendering block.

What to change
In Agent.py:create_visualizations, wrap the _save calls (or
the per-chart plt.subplots / plt.figure blocks) with
plt.style.context(...). The two-line setup at lines 942-943
becomes a single with plt.style.context('default'): sns.set_palette("husl")
block scoped to the rendering loop, or you can drop the
set_palette entirely if the default husl is acceptable.

How to verify

• Import matplotlib in a separate script, change the user's
  rcParams to a custom style, then call
  agent.create_visualizations(df, "f.csv"). After the call,
  the custom rcParams must still be in effect.
• python -m unittest discover -s tests -v is green; specifically
  the CreateVisualizationsGuardTests continue to produce the
  expected chart types.

Skill: matplotlib, scoping.
Estimated effort: S.