feat(tokens): add offline client-token verification

[AdirAmsalem]

Client tokens from client.tokens.create are EdDSA JWTs verifiable offline against the platform JWKS, but the SDK had no way to verify them. This adds it.

API

verified = await client.tokens.verify(token)   # or: await verify_client_token(token)
verified.service_tier, verified.pool, verified.user_id, verified.expires_at

claims = client.tokens.decode(token)            # or: decode_client_token(token) — no verification, untrusted

• Checks the EdDSA signature against https://platform.decart.ai/api/auth/jwks (fetched once and cached via PyJWT's PyJWKClient), plus exp (30s leeway), iss and aud. jwks_url, issuer, audience, leeway are overridable.
• Returns VerifiedClientToken; decode returns its untrusted parent ClientTokenClaims. Fields: user_id, organization_id, api_key_id (parent key, else token id), api_key_name, service_tier, pool ("free" for tier 0, else "paid"), allowed_models, allowed_origins, constraints, realtime_concurrent_session_limit, zero_data_retention, attribution, expires_at, raw claims.
• Raises TokenVerifyError / TokenDecodeError.
• Requires the new optional extra decart[verify] (pyjwt[crypto]), imported lazily so minting-only users pull no crypto dependency. decode needs no extra.

---

Note

Medium Risk
Introduces a security-sensitive JWT verification path backends may use for authorization; behavior is well-tested and optional, but incorrect adoption of decode without verify could weaken trust assumptions.

Overview
Adds offline verification and decoding for short-lived client JWTs minted via client.tokens.create, so backends can trust signed claims (tier, models, origins, expiry) without calling the platform.

New verify_client_token / client.tokens.verify validate EdDSA signatures against the platform JWKS (https://platform.decart.ai/api/auth/jwks, cached in-process), plus exp (30s leeway), iss, and aud. decode_client_token / client.tokens.decode parse claims with no signature check (untrusted). Results are typed as VerifiedClientToken / ClientTokenClaims with mapped fields including pool ("free" for tier 0). Failures raise TokenVerifyError / TokenDecodeError.

Verification is gated behind optional decart[verify] (pyjwt[crypto]), lazily imported so token creation stays lightweight. README documents the flow; a large test suite covers tampering, expiry, JWKS caching, and key rotation.

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