fix: sanitize underscores in agent worker suffix (#8992)

* fix: sanitize underscores in agent worker suffix

The agent worker token wire format is `jwt_agent_<suffix>_<JWT>`, parsed
server-side with `split_once('_')`. JWTs themselves can contain `_`
(base64url alphabet), so the only sound boundary is "suffix has no `_`".

`instance_name()` is the source of the suffix and previously only
sanitized spaces and `-`. A hostname like `austin_hp` would produce
`worker_suffix=austin_hp-<rand>`, the token `jwt_agent_austin_hp-X_<JWT>`
would split as `("austin", "hp-X_<JWT>")`, and the JWT decoder would
return `InvalidToken` on the garbage second half, surfacing as a 401 on
`/api/agent_workers/update_ping`.

Replace `_` with `-` in the hostname-derived suffix so the parsing
boundary stays unambiguous. Pure source-side fix; no wire format change,
no migration needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix: apply underscore->dash before splitting in instance_name

Replace `_` with `-` BEFORE the `split("-").last()` step so underscores
behave the same as dashes (consistent with the split-on-dash idiom) and
the resulting instance_name is a single token. For `austin_hp` this
yields `hp` rather than `austin-hp`. No behavior change for hostnames
without `_`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix: strip underscores from hostname instead of replacing with dash

Removing `_` preserves more identifier info than replacing with `-`:
`austin_hp` becomes `austinhp` (single useful token) rather than `hp`
(prefix lost to the dash-split). For k8s-style hostnames that already
contain `-`, the `-` continues to do the splitting and `_` is just a
stray character to strip.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/windmill-common/src/utils.rs

@@ -259,11 +259,11 @@ pub async fn require_admin_or_devops(
 fn instance_name(hostname: &str) -> String {
     hostname
         .replace(" ", "")
+        .replace('_', "")
         .split("-")
         .last()
         .unwrap()
         .to_ascii_lowercase()
-        .to_string()
 }
 
 const DEFAULT_WORKER_SUFFIX_LEN: usize = 5;