[expression] Vectorized ADDTIME can accept a datetime duration and overwrite rows that scalar execution rejects

## Bug Report

### 1. Minimal reproduce step (Required)

**Environment**

TiDB 13282a8bd0; TiKV 730be34f; PD f7db4252; default vector+strict+MDL; real TiKV; no injection

**Steps**

Create a VARCHAR source with base datetime strings and delta strings 2024-01-02 03:04:05, 0000-00-00 01:02:03, and valid 01:02:03. Seed scalar and vector destination copies with NOT NULL DATETIME values and payload markers. Run identical REPLACE INTO dst SELECT id,ADDTIME(base,delta),new with tidb_enable_vectorized_expression OFF and ON in separate sessions.

### 2. What did you expect to see? (Required)

Scalar and vectorized ADDTIME enforce the same pure-duration domain for the second string argument. Both strict writes return 1048 on the first NULL result and preserve every destination preimage.

### 3. What did you see instead? (Required)

Scalar ADDTIME returns NULL for both date-bearing delta strings, so strict REPLACE returns 1048 and preserves all four rows. Default vectorized ADDTIME returns 2024-01-01 03:04:05 and 2024-01-02 01:02:03, reports success with zero warnings, and replaces the two protected rows. The valid duration control is updated in both semantics.

### 4. What is your TiDB version? (Required)

TiDB 13282a8bd0; TiKV 730be34f; PD f7db4252; default vector+strict+MDL; real TiKV; no injection

Likely root cause and fix direction

**Likely root cause**

After ParseDuration succeeds, builtinAddStringAndStringSig.evalString applies a second parser check that rejects number-hyphen-remainder input because it belongs to the datetime domain. The generated vector implementation omits that post-parse classifier and assumes ParseDuration success proves a pure duration. SUBTIME shares the same generated omission.

**Fix direction**

Copy the scalar date-part rejection into the shared AddTime/SubTime vector generation template immediately after string-to-duration parsing. Add scalar/vector parity over date-bearing and pure-duration VARCHAR values plus a strict NOT NULL REPLACE preimage oracle.

Start with builtinAddStringAndStringSig.evalString and the shared AddTime/SubTime vector generation template, then run the minimal scalar/vector ADDTIME reproduction from the issue. Done means date-bearing duration strings have matching scalar and vector behavior, with parity coverage for date-bearing and pure-duration VARCHAR values and the strict NOT NULL REPLACE preimage oracle.

Written by the indexing model from the issue text.