Tiny documentation styling fixes [ci skip]
Pass the `SelectManager`, rather than the AST when querying w/ Relation

Arel specifically handles `SelectManager`, with the same logic we're
currently performing. The AST is `Enumerable`, which Arel looks for
separately now.
Deprecate `sanitize_sql_hash_` methods on ActiveRecord

These appear to be implementation relics of times past. They duplicate
the logic in Relation, and are no longer used internally.
Revert deprecation of `sanitize_sql_hash_for_assignment`

This method is still used by `update_all`
Handle `RangeError` from casting in `find_by` and `find_by!` on Relation

We should not behave differently just because a class has a default
scope.
Add an `assert_deprecated` for `sanitize_sql_hash_for_conditions`
Revert "Replace String index juggling with Pathname goodness in db:fixtures:load"

This reverts commit 482fdad5ef8a73688b50bba3991dd4ef6f286edd.

Fixes #17237.
Add comment to point out String#[] is intentional