Rebuild one function's runtime from stored source

Re-bundles a single function from its STORED source against the gateway's current entry wrapper / bundled runtime, with dependencies pinned to the function's exact recorded deps_resolved versions — so the only change is the wrapper/runtime (the source code_hash is unchanged and no new release is created). Use it to pick up a platform-side wrapper/runtime fix without editing your source. Strictly opt-in — functions are never auto-rebuilt. Wallet-authed and ownership-checked; identity only — NO active tier is required, so it works during billing grace (past_due/frozen/dormant). A failed rebuild leaves the old artifact and DB row intact.