fix: enforce TINYTEXT byte length limit

[iamlinjunhong]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #26687

What this PR does / why we need it:

fix: enforce TINYTEXT byte length limit

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[XuPeng-SH]

Request changes: provide an upgrade path for existing TINYTEXT columns.

Before this PR, getTypeFromAst persisted every TEXT-family declaration, including TINYTEXT, as T_text with Width=0. This PR recognizes TINYTEXT only while parsing new DDL and all enforcement/formatting helpers key exclusively on T_text + Width=255.

Consequently, a TINYTEXT table created on any pre-fix version still resolves as Width=0 after upgrade. assignmentCastFunctionName then deliberately selects ordinary cast (the new test even asserts this for T_text/Width=0), so the original 1000-byte reproduction continues to store 1000 bytes. SHOW CREATE and CREATE TABLE ... LIKE also treat/copy that legacy column as unbounded TEXT. The fix therefore applies only to tables created or explicitly rebuilt after deployment.

Please add a durable subtype recovery/migration path (for example, versioned catalog metadata/backfill, or a carefully validated recovery from TableDef.Createsql) and define what happens when legacy columns already contain oversized rows. Add an upgrade regression that starts from a pre-fix catalog definition and verifies future strict/non-strict writes plus SHOW/LIKE behavior. The new-table byte-boundary, assignment-mode, DEFAULT, and formatting tests all pass on exact head a9dae6d80b97; the missing piece is existing-schema compatibility.