Restructure ReadExpr: encode expression/wrapper validity in types

The remote client's ReadExpr is a flat enum; the relationships between what is a valid input to one expression and its wrapping expression are lost, and validity is enforced ad hoc at render/parse time. Restructure so invalid trees are unrepresentable (typed stages or a builder), which also removes a class of runtime InvalidResponse/InvalidInput checks.

In scope: each terminal's shape is declared three times

A related symptom of the same flat structure: every terminal's wire shape lives in three independent places that must agree, and a mismatch compiles fine and only fails at runtime.

Worked example — changing earliestEdgeTime from a bare i64 to an EventTime required editing all three:

// 1. render — which sub-fields to select
out.push_str(" { earliestEdgeTime { timestamp eventId");

// 2. path builder — how deep to descend into the response
out.push("earliestEdgeTime");
out.push("timestamp");        // right for a scalar, wrong for an object

// 3. parse arm — how to decode what is found there
terminal_val.as_i64()         // wrong once the field is an object

Updating (1) alone produced two successive runtime errors, one per missed site: Invalid response: `earliestEdgeTime` returned unexpected value type, then Invalid response: `timestamp` not a JSON object.

Note a shared field_name() helper would not catch this, since none of the three mistakes is a misspelled name — render_read_into has ~34 field literals and build_json_path ~137 path segments, but the bug class is the shape, not the string. The fix is to declare each terminal once (name + path depth + decoder) so "scalar int" versus "EventTime object" is a single decision the three arms derive from.

Raised in review: https://github.com/Pometry/Raphtory/pull/2675#discussion_r3804975599

Start with the remote client's ReadExpr and the render_read_into and build_json_path entry points named in the issue. Trace how terminal values are rendered, located, and decoded, then design a typed or builder-based structure that derives these shapes from one declaration. Done means invalid expression trees are unrepresentable and the duplicated terminal-shape logic and corresponding runtime validity checks are removed.