prototype(gate_store): reject float overflow, out-of-range integers, empty identifiers, and invalid RemoteGateStore timeout #645

What is wrong

On main at b938790, prototype/gate_store.py has four input-validation gaps across the wire protocol and client/server boundaries:

1. Float exponent overflow in _recv: _recv passes parse_constant=_no_constants to json.loads to reject literal NaN/Infinity, but omits parse_float, so overflowed float literals such as 1e309 or -1e309 parse to inf/-inf without raising ValueError.
2. Uncaught OverflowError on 64-bit integer overflow in _int / _Handler.handle: _int accepts any Python int, including negative values or values >= 2**63 (or > 9007199254740991). When passed to SQLite in mint_challenge, sqlite3 raises built-in OverflowError (Python int too large to convert to SQLite INTEGER), which is neither a (KeyError, TypeError, ValueError) nor a sqlite3.Error, so _Handler.handle fails to catch it and drops the connection instead of returning {"error": "malformed"}.
3. Empty string identifiers and unvalidated nonce format in _str / _dispatch: _str accepts "" for issuer, token_id, session_id, and call_id, and _dispatch accepts non-hex or wrong-length nonce values instead of requiring 32 lowercase hex characters ([0-9a-f]{32}).
4. Unvalidated timeout in RemoteGateStore.__init__: RemoteGateStore accepts timeout=False, timeout=0, timeout=-1.0, or timeout=float("nan") at construction time instead of raising ValueError.

Spec section or file

prototype/gate_store.py (_recv, _str, _int, _dispatch, RemoteGateStore.__init__)

Expected behavior

• _recv rejects non-finite parse_float values with ValueError.
• _int enforces 0 <= value <= v._MAX_INT (9007199254740991), raising ValueError on negative or oversized integers so _Handler.handle returns {"error": "malformed"}.
• _str rejects empty strings "", and _nonce requires [0-9a-f]{32}.
• RemoteGateStore.__init__ validates that timeout is a positive finite int or float (excluding bool).

Impact

Prevents unhandled OverflowError crashes in GateStoreServer, non-finite float smuggling over the wire protocol, empty identifier rows in SQLite state, and invalid socket timeouts in RemoteGateStore.