feat: add Dynamic Highlights (research preview) via betas param

[kesku]

Adds dynamic: bool to HighlightsContentsOptions and an explicit betas param to search / get_contents (sync + async) that sends the Exa-Beta header, following the agent betas convention. Exports DYNAMIC_HIGHLIGHTS_BETA = "dynamic-highlights-2026-08-28". No header is sent unless betas is passed; stable calls are unchanged.

from exa_py import Exa
from exa_py.api import DYNAMIC_HIGHLIGHTS_BETA

exa = Exa()

results = exa.search(
    "agentic retrieval",
    contents={"highlights": {"dynamic": True}},
    betas=[DYNAMIC_HIGHLIGHTS_BETA],
)

contents = exa.get_contents(
    ["https://example.com"],
    highlights={"dynamic": True},
    betas=[DYNAMIC_HIGHLIGHTS_BETA],
)