Type SESSION_PROPOSE config with a validated SessionProposeConfig model (#18)

[phillipclapham]

Closes #18. Rebased onto main now that #17 has landed, so the diff is just the typed-config change.

Replaces the config: dict[str, Any] hatch on LdpMessageBody with a typed SessionProposeConfig, so Pydantic enforces the wire shape at the boundary:

class SessionProposeConfig(BaseModel):
    preferred_payload_modes: list[PayloadMode] = Field(default_factory=list)
    ttl_secs: int | None = Field(default=None, gt=0)
    trust_domain: str  # required, non-blank

• preferred_payload_modes keeps the forward-compat tolerance the manual guards from Mode 3 (SEMANTIC_GRAPH) negotiation via declared support + codec registration (#15) + lifecycle hooks (#14) #17 added — a mode="before" validator drops non-list / unknown / non-string entries, so a peer on a newer mode set degrades to the shared known modes instead of failing the handshake. Those manual guards are retired from the handler.
• trust_domain is required and non-blank (validator rejects "" / whitespace, mirroring TrustDomain.name). Deliberate contract tightening: previously a SESSION_PROPOSE missing trust_domain (or with no config) was treated as the "unknown" domain; now it's rejected at the boundary, and a model_validator requires config for SESSION_PROPOSE so it holds end to end. The bundled client always sends trust_domain, so it's unaffected. If you'd rather keep the lenient default, trust_domain: str = "unknown" is a one-line change.
• ttl_secs, when present, must be positive.

One design question: LdpMessageBody is a flat model, so the typed config field is validated on every message type, not just SESSION_PROPOSE — a future message carrying a config that isn't a valid SessionProposeConfig would be rejected at the boundary rather than reaching the unknown-message path. That's consistent with how the model already validates every other typed field (negotiated_mode, contract, …), and no current message carries config elsewhere, so there's no blast radius today. If you'd want config to stay a universal extension hatch, the clean options are a discriminated union or conditionally coercing config only when type == "SESSION_PROPOSE" — your call on evolvability, happy to implement either.

Heads-up for the Rust mirror: Python conveys SESSION_PROPOSE mode prefs as config.preferred_payload_modes, but the Rust server reads a top-level payload_mode (src/server.rs), so Python↔Rust mode negotiation looks like it diverges today (predates this PR). Worth a cross-SDK JSON fixture when you align Rust — happy to file a separate issue.

Tests cover: drops unknown modes, coerces strings, requires non-blank trust_domain, rejects non-positive ttl, requires config for SESSION_PROPOSE, validates through the envelope boundary, and the factory dict path enforces the same. Also fixed examples/session_flow.json, which used required_trust_domain (a client-config field name) in the wire config.

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• examples/session_flow.json
• sdk/python/src/ldp_protocol/client.py
• sdk/python/src/ldp_protocol/delegate.py
• sdk/python/src/ldp_protocol/types/__init__.py
• sdk/python/src/ldp_protocol/types/messages.py
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