Fixed #36030 -- Avoid unnecessary CAST(... AS NUMERIC) for Value(Decimal) on SQLite.

[VIZZARD-X]

Trac ticket number

ticket-36030

Branch description

SQLite already preserves Decimal values without requiring an explicit
CAST(... AS NUMERIC). However, Django currently applies this CAST
unconditionally in SQLiteNumericMixin, even when the expression is
a simple Value(Decimal).

As reported in ticket #36030, this produces redundant SQL and causes
unnecessary type coercion on SQLite.

Following the discussion on the ticket, this patch updates
SQLiteNumericMixin.as_sqlite() to skip numeric casting when the
expression is Value(Decimal), matching SQLite's native behavior.

This change:

• avoids unnecessary SQL casting,
• preserves correct numeric behavior,
• keeps the existing logic for all other DecimalField expressions,
• requires no changes for other backends.

No new tests are required (ticket metadata: Needs tests: no).

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" flag on the Trac ticket.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in light/dark modes for UI changes (N/A).

[github-actions]

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