[Advanced] Python client for contextio-sdk (feature parity with the TS SDK)

[Eras256]

Context
contextio-sdk (npm, TypeScript — packages/sdk/src/client.ts) is the only official client today. No client exists in any other language, which blocks any Python backend/script from integrating without hand-rolling HTTP calls.

Scope
A Python package mirroring the real, current surface of ContextioClient:

• challenge(address: str) -> WalletChallenge
• verify(address, signed_message, ...) -> WalletSession (same input shape as the TS verify())
• with_session(session) — client bound to an existing session, no re-auth
• treasury(), obligations(), decisions(), propose(), legal()
• well_known_legal_context(domain) (mirrors wellKnownLegalContext)

SEP-53 signing itself is out of scope — accept a caller-supplied signing callback, same pattern the JS client expects from its caller. Do not reimplement Stellar keypair signing inside the client.

Acceptance criteria

• Installable (pip install or poetry add from this repo, e.g. packages/sdk-python/).
• A runnable example script completes challenge → verify → treasury() → obligations() against the live public testnet demo API (https://contextio-api.fly.dev).
• Unit tests cover request/response shapes with mocked HTTP — no live network required for CI to pass.
• README documents install + the runnable example.

References
packages/sdk/src/client.ts, packages/sdk/src/types.ts for the exact shapes to mirror.