fix(start-client-core): safely serialize Standard Schema validation issues

Problem

Standard Schema validators can return issue objects that cannot be serialized with JSON.stringify. This hides the original validation failure.

The same raw serialization pattern exists in server-function validation and router search validation.

Scope

Create a shared internal formatter for Standard Schema issues and use it in both paths.

The formatter must:

• Preserve issue messages.
• Handle root and nested paths without ambiguous dot-path collisions.
• Safely handle prototype-named keys such as __proto__.
• Avoid adding a public router-core export unless there is a supported external use case.

Tests

• Add unit coverage for root issues, nested paths, numeric keys, special property names, and non-serializable issue objects.
• Add E2E coverage for server-function validation using multiple Standard Schema-compatible libraries, as requested in #3708.
• Verify router search validation uses the same safe formatting behavior.
• Run the relevant React Start bundle-size scenario.

Documentation

Review and update the validator documentation to describe Standard Schema compatibility and the resulting validation-error behavior where user-facing.

Related work

Supersedes stale #3708 which should be updated for the current validation architecture and cover both server-function and router search validation.

Start by locating the raw JSON.stringify paths in server-function validation and router search validation, then review the current validation tests and the stale #3708 requirements. Add coverage for root and nested paths, numeric and special property names, and non-serializable issues, plus the requested server-function E2E cases. Done means both validation paths share safe formatting, user-facing validator documentation is updated, and the relevant React Start bundle-size scenario passes.