[expression] Vectorized CONVERT_TZ can silently overwrite rows from incomplete dates under default strict mode

## Bug Report

### 1. Minimal reproduce step (Required)

**Environment**

TiDB master 05b396fb66; TiKV 730be34f; default vector ON; global strict mode; MDL ON; one TiDB/PD/TiKV; no injection

**Steps**

Create a DATE source under permissive sql_mode with 2024-00-15, 2024-01-00, and 2024-01-15. Seed scalar and vector destination copies with valid timestamps and payloads. Restore the global default strict sql_mode. Run identical REPLACE INTO dst SELECT id,CONVERT_TZ(d,+00:00,+08:00),new with tidb_enable_vectorized_expression OFF and ON in separate sessions. Compare terminals and full destination rows.

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

Scalar and vectorized evaluation preserve the same invalid-time warning policy. Under default strict DML both statements return 1292 and preserve every seeded destination preimage.

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

Scalar execution returns 1292 on 2024-00-15 and preserves all three rows. Default vectorized execution returns success; rows 1 and 2 become NULL/new and row 3 becomes 2024-01-15 08:00:00/new. Two existing valid timestamp and payload preimages are silently lost.

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

TiDB master 05b396fb66; TiKV 730be34f; default vector ON; global strict mode; MDL ON; one TiDB/PD/TiKV; no injection

Likely root cause and fix direction

**Likely root cause**

builtinConvertTzSig.evalTime rejects Time.InvalidZero through handleInvalidTimeError. builtinConvertTzSig.vecEvalTime directly calls convertTz, which returns NULL for the same incomplete values without appending the warning. The missing warning crosses the default strict-DML boundary and changes statement abort into commit.

**Fix direction**

Copy the scalar InvalidZero plus handleInvalidTimeError branch into vecEvalTime before convertTz. Add scalar/vector parity for value, NULL, warnings, and strict INSERT/REPLACE terminals over incomplete month/day values.

Start by comparing builtinConvertTzSig.evalTime with builtinConvertTzSig.vecEvalTime and reproduce the scalar/vectorized CONVERT_TZ cases described in the issue under strict DML. Verify that incomplete month/day values produce matching values, NULL results, warnings, and INSERT/REPLACE outcomes; done means vectorized execution preserves the same destination rows and error behavior as scalar execution.

Written by the indexing model from the issue text.