MDEV-38329 Named parameters in stored procedure CALL

[MooSayed1]

The Jira issue number for this PR is: MDEV-38329

Description

Stored procedures currently require all arguments to be passed positionally.
When a routine has many parameters with defaults, there is no way to skip
middle parameters — only trailing ones can be omitted.

This patch adds named parameter invocation using the => syntax:

CALL proc(a => 1, b => 2, c => 3);       -- all named
CALL proc(c => 3, a => 1, b => 2);       -- any order
CALL proc(1, b => 2, c => 3);            -- mixed positional + named
CALL proc(a => 1, c => 3);               -- skip middle param with default

This brings MariaDB in line with Oracle, PostgreSQL, SQL Server, and Firebird,
all of which already support named parameter invocation.

Parser changes (sql/sql_yacc.yy):

Added sp_cparam rule to accept ident ARROW_SYM expr alongside plain expr
in CALL argument lists. Named arguments set IS_EXPLICIT_NAME on the Item and
store the parameter name in Item::name, reusing the same mechanism that UDF
named arguments already use. Positional arguments after a named argument are
rejected at parse time.

Reordering (sql/sp_head.cc):

In sp_head::execute_procedure(), before the binding loop, named arguments are
matched against sp_pcontext's formal parameter list by name and reordered to
their declared positions. Omitted parameters with defaults are filled from
sp_variable::default_value. The following errors are detected:

• Unknown parameter name (ER_SP_UNDECLARED_VAR)
• Duplicate parameter name (ER_SP_UNDECLARED_VAR)
• Missing required parameter without default (ER_SP_WRONG_NO_OF_ARGS)

Current status

Done:

• Parser: CALL proc(a => 1, b => 2) syntax accepted
• Mixed positional + named: CALL proc(1, b => 2)
• Parse-time rejection of positional after named
• Argument reordering in execute_procedure()
• Default value filling for omitted parameters
• Error detection: unknown name, duplicate name, missing required param
• MTR test coverage

Still working on:

• Named params for stored functions in expressions (SELECT func(a => 1))
• Prepared statement support (PREPARE stmt FROM 'CALL p(a => ?)')
• OUT/INOUT parameter support with named args
• Extended test coverage (edge cases, nested calls, reserved words as param names)

This PR is a work in progress. Reviews and feedback on the current approach are welcome.

Release Notes

Stored procedures now support named parameter invocation using the => syntax.
Arguments can be passed by name in any order, and parameters with default values
can be skipped: CALL proc(a => 1, c => 3).

How can this PR be tested?

./mtr main.sp_named_params

The test covers:

• All positional (no regression)
• All named in declared order and different order
• Mixed positional + named
• Positional after named (syntax error)
• Unknown parameter name, duplicate name
• Default value: skip middle param, only required param, override all defaults
• Missing required parameter without default

Basing the PR against the correct MariaDB version

This is a new feature. The PR targets main.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.