fix: replace conflicting registrations instead of aborting the LOAD

[jrosskopf]

Registering a name the catalog already holds aborts the whole extension load. ExtensionLoader::RegisterFunction(PragmaFunction) and the CreateTableFunctionInfo overload both leave CreateInfo::on_conflict at its ERROR_ON_CONFLICT default. (The TableFunctionSet overload sets ALTER, which is why this only bites the registrations that build their own info in order to attach a FunctionDescription.)

Why now

erpl is dropping its bundled SSH tunnel (DataZooDE/erpl#119) and leaving deprecation stubs under these same names, pointing callers here. Without this change, a user who does LOAD erpl, hits the stub, and follows its advice:

INSTALL erpl_tunnel FROM 'http://get.erpl.io';
LOAD erpl_tunnel;

gets:

Invalid Input Error: Initialization function "erpl_tunnel_duckdb_cpp_init" threw an
exception: "Pragma Function with name "tunnel_create" already exists!"

The migration path the message recommends was the one path that did not work.

Verified against a real erpl build with the stubs present

• Before: LOAD erpl_tunnel aborted on tunnel_create.
• After fixing only the pragmas: it aborted later, on the tunnels table function, leaving the extension half-registered — loaded = false, and tunnel_peers/tunnel_self silently missing. That second failure is the reason the table-function registrations are included here; fixing the pragmas alone looks like it works until you count the functions.
• After both: either load order — erpl then erpl_tunnel, or the reverse — registers all nine functions, and PRAGMA tunnel_close_all is served by the real implementation.

Why REPLACE and not IGNORE

This extension owns these names. IGNORE_ON_CONFLICT would leave erpl's stub in place, so the name would resolve to something that only knows how to raise "this moved" — worse than the abort, because it would look like it worked.

No behaviour change when no other extension holds the names, which is every case except the migration window.

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