feat: Make VALIDATE clause optional when creating webhook connection

[cnsky1103]

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What's changed and what's your intention?

#21596

Made webhook validation optional by defaulting webhook metadata even when VALIDATE is omitted and skipping signature checks when no expression is provided.

dev=> create table wbhtable (
  data JSONB
) WITH (
  connector = 'webhook');
CREATE_TABLE

dev=> show create table wbhtable;
      Name       |                           Create Sql
-----------------+-----------------------------------------------------------------
 public.wbhtable | CREATE TABLE wbhtable (data JSONB) WITH (connector = 'webhook')
(1 row)

curl -v -X POST \
  http://localhost:4560/webhook/dev/public/wbhtable \
  -H 'Content-Type: application/json' \
  -d '{"data":"abc"}'

< HTTP/1.1 200 OK
< Content-Length: 0
< Access-Control-Allow-Origin: *
< Connection: keep-alive
< Date: Mon, 15 Dec 2025 02:19:04 GMT
< Keep-Alive: timeout=4
< Proxy-Connection: keep-alive
< Vary: origin, access-control-request-method, access-control-request-headers
<
* Connection #0 to host 127.0.0.1 left intact

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note

[BugenZhao]

How about suggesting to users (via notice_to_user) that a VALIDATE clause is strongly recommended for safety or production usages?

[hzxa21]

+1

[Chiro11]

+2

[cnsky1103]

message added

[hzxa21]

LGTM.

@Chiro11 PTAL as well.

[Chiro11]

LGTM.