Fix array positions rendering as object keys, and subscripts served from the statement cache

[bgunebakan]

Summary of the changes / Why this is an improvement

CrateDB rejects a bare bind parameter as a position or key (arr[?] fails with Parameter substitution is not supported in subscript index), but accepts one inside an expression (arr[id + ?], arr[abs(?)], also as an UPDATE target).

• An integer as a position, a string as a quoted, escaped key. When the statement is compiled for the cache, the literal is filled in on each execution (literal_execute).
• An expression that renders as a bare parameter, like type_coerce(literal(2), Integer), is unwrapped and treated as a plain value.
• Slices render as start:stop, with bind parameters as bounds. A step raises CompileError.
• An integer on a top-level object keeps meaning the key (obj[1] → obj['1'], as on main), because CrateDB never accepts a position there.
• A subscript that is neither an integer nor a string raises CompileError.

Behavior changes

• ObjectArray and sa.JSON subscripts no longer return a cached statement's results.
• bool, float and None subscripts raise CompileError. Before, they rendered as keys such as obj['True'].
• A cached statement with a literal subscript, like WHERE js['x'] = :v, can't run with executemany(): SQLAlchemy doesn't support literal_execute there, the same restriction it has for expanding IN.

References for implementation

• https://docs.sqlalchemy.org/en/20/core/connections.html#caching-for-third-party-dialects
• https://github.com/sqlalchemy/sqlalchemy/blob/4d80275bd425a07be59ad41062b0d58120bd41da/lib/sqlalchemy/dialects/mssql/base.py#L2113-L2126
• https://github.com/sqlalchemy/sqlalchemy/blob/4d80275bd425a07be59ad41062b0d58120bd41da/lib/sqlalchemy/dialects/oracle/base.py#L1550

Out of scope

• tests/query_caching.py renamed to tests/query_caching_test.py to align with naming on all tests.

Checklist

• Link to issue this PR refers to (if applicable): Fixes Reading an array element by position fails #309

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[florinutz]

PR looks ace, but I think you just need to signal more changes:

Suggested change

	- Compiler: Fixed array indexes rendering as object keys, as in `arr['1']`.
	Added support for array slices, and for columns and expressions as indexes
	- Fixed the statement cache returning results for another key on
	`ObjectArray` and `JSON` subscripts and escaped quotes in object keys.
	- Statements with a literal subscript, as in `js['x']` run with
	`execution_options(compiled_cache=None)`
	- Compiler: Fixed array indexes rendering as object keys, as in `arr['1']`.
	Added support for array slices, and for columns and expressions as indexes
	- Compiler: Fixed cached statements on `sa.JSON` and `ObjectArray` columns
	reusing the subscript of an earlier statement, which returned values of
	another key. Fixed object keys containing a quote
	- Compiler: A statement with a literal subscript can't run with `executemany()`
	on a cached engine. For that, use `execution_options(compiled_cache=None)`
	- Compiler: An integer subscript on a nested object renders as an array
	position. Write the key as a string, as in `obj['n']['1']`
	- Compiler: `bool`, `float` and `None` subscripts raise `CompileError`