fix: don't sum row counts across shards for EXECUTE on omnisharded tables

[mscrivo]

Problem

EXECUTE of a server-side prepared statement targeting an omnisharded table returned the sum of row counts from all shards instead of the count from one shard:

PREPARE upd AS UPDATE sharded_omni SET value = $1;
EXECUTE upd('x');  -- UPDATE 6 on a 2-shard cluster with 3 rows (should be UPDATE 3)

Plain UPDATE/DELETE/INSERT on omnisharded tables already report the correct count (#752), but ExecuteStmt fell through to the DDL catch-all, which broadcasts with Route::write(Shard::All) and no omnisharded flag — so MultiShard aggregated the CommandComplete counts.

Fix

• Route PREPARE explicitly: still broadcast to all shards, but record the statement behind the name in the client's PreparedStatements state (new simple map, cleared with close_all()).
• Route EXECUTE explicitly: resolve the name — first from the recorded SQL-level PREPAREs, falling back to the global cache for names rewritten by prepared_statements = "full" — and set the omnisharded flag on the route when the underlying statement only touches omnisharded tables. MultiShard then deduplicates results across shards instead of aggregating them.
• The router gets access to the client's prepared statements via a new optional RouterContext::with_prepared_statements().

This also extends the omni-write-in-direct-to-shard-transaction guard (#1086) to EXECUTE of omnisharded writes, since that check keys off the same route flag.

Testing

• New routing tests in parser/query/test/test_execute.rs (fail without the fix): EXECUTE of omnisharded UPDATE/DELETE carries the omnisharded flag; sharded/unknown statements don't; global-name resolution works.
• Verified end-to-end against a 2-shard cluster: EXECUTE upd('x') now returns UPDATE 3 instead of UPDATE 6, and the write still reaches every shard. Same for DELETE.
• cargo nextest parser suite, cargo clippy, cargo check --features new_parser all pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 94.23077% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/parser/query/execute.rs	90.76%	6 Missing ⚠️
pgdog/src/frontend/prepared_statements/mod.rs	84.61%	2 Missing ⚠️
...og/src/frontend/client/query_engine/route_query.rs	50.00%	1 Missing ⚠️

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[sgrif]

This touches the parser, which we are in the middle of a major rewrite of. This isn't likely to get reviewed until after that is done (we are hoping next week, but take that with a huge grain of salt)