fix(cross_shard_disabled): provide more specific error msg when dealing with unknown sharding key

[jkaczman]

Provides a specific error message "unknown sharding key was specified" as opposed to "cross-shard queries are disabled" when a query:

1. has an unknown sharding key set (list-based / range-based)
2. cross_shard_disabled=true
3. would be cross-shard if not stopped

Fixes #1163

[levkk]

I think this is fine for now. A better solution could be to check this inside the parser:

1. If it returns Shard::All
2. and pgdog.sharding_key is set in params (those are available in the context, see

   pgdog/pgdog/src/frontend/router/parameter_hints.rs

   Line 29 in 4bafac9

   pub pgdog_sharding_key: Option<&'a ParameterValue>,

   )
3. and cross-shard queries are disabled (this may be currently missing from the context but can be easily added I think:

   pgdog/pgdog/src/frontend/router/parser/context.rs

   Line 21 in 4bafac9

   pub struct QueryParserContext<'a> {

   )

then the parser can return this error, which we can catch and send to the client.

The reason this is better is because it avoids connecting to Postgres when we know we can't serve that query because of the missing sharding key mapping.

[levkk]

Another reason why the parser could be a better place for this is because it'll also catch the comment directive:

/* pgdog_sharding_key: 25 */ SELECT * FROM users;

while this check will only catch the pgdog.sharding_key SET parameter.

[codecov]

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