feat: add helpers to fetch all markets

[danytrackmania]

Summary

This PR adds two convenience methods to ClobClient:

• get_all_markets
• get_all_simplified_markets

Both methods iterate through the paginated CLOB API using the next_cursor and
END_CURSOR convention and return a flat list of all markets.

Motivation

Currently, get_markets and get_simplified_markets only return a single
page of results and require client code to implement pagination manually.
These helpers provide a simple, ergonomic way to fetch all markets without
breaking the existing API surface.

Implementation details

• get_all_markets calls get_markets in a loop and aggregates response["data"]
  until next_cursor == END_CURSOR.
• get_all_simplified_markets does the same using get_simplified_markets.
• No existing methods were modified; this is a purely additive change.

Testing

Manually tested against the CLOB API by verifying that:

• all pages are fetched until END_CURSOR,
• the length of the aggregated result matches the total across pages.

---

Note

Introduce get_all_markets and get_all_simplified_markets to iterate pagination and return full market lists; clarify and type-hint single-page get_markets and get_simplified_markets.

• Client (py_clob_client/client.py):
  • New pagination helpers:
    • get_all_markets(next_cursor: str = "MA=="): loops through cursors until END_CURSOR, aggregates response["data"].
    • get_all_simplified_markets(next_cursor: str = "MA=="): same for simplified markets.
  • API clarifications + typing:
    • Add str type hints and clarify docs for get_markets and get_simplified_markets as single-page fetches.

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