Cron delivery fails for Enterprise Slack workspaces — is_valid_instance_name misidentifies workspace IDs

Summary

Enterprise Slack workspace/channel IDs (e.g. T0APRHSB676, D0APL3DF66S, C0APL3DF66S) contain mixed uppercase letters and digits. The is_valid_instance_name() function in src/messaging/target.rs only rejects the classic format (uppercase letter + all-digits, like T012345), causing Enterprise IDs to be misidentified as named adapter instances.

Impact

All cron jobs created from Enterprise Slack workspaces fail delivery. The stored delivery target slack:T0APRHSB676:D0APL3DF66S is parsed as:

• Expected: adapter=slack, target=D0APL3DF66S
• Actual: adapter=slack:T0APRHSB676, target=D0APL3DF66S

This causes broadcast() to fail with no messaging adapter named 'slack:T0APRHSB676'.

Regular (non-cron) messages are unaffected because they use the adapter reference from the inbound message object, bypassing the delivery target parser.

Root Cause

is_valid_instance_name() at line 670:

// Only rejects: uppercase letter + ALL digits (T012345)
if name.len() > 6
    && name.starts_with(|c: char| c.is_ascii_uppercase())
    && name[1..].chars().all(|c| c.is_ascii_digit())

Enterprise IDs like T0APRHSB676 have mixed uppercase+digits in the tail, so they pass through as valid instance names.

Fix

Broaden the pattern to also reject uppercase letter + uppercase/digits when length >= 9:

if name.starts_with(|c: char| c.is_ascii_uppercase()) {
    let tail = &name[1..];
    if (name.len() > 6 && tail.chars().all(|c| c.is_ascii_digit()))
        || (name.len() >= 9 && tail.chars().all(|c| c.is_ascii_uppercase() || c.is_ascii_digit()))
    {
        return false;
    }
}

Fix committed on branch fix/upgrade-slack-morphism with tests.

Start in src/messaging/target.rs at is_valid_instance_name(), then review the existing tests for delivery-target parsing. Verify that Enterprise Slack workspace and channel IDs are not treated as named adapter instances, and confirm cron delivery succeeds for targets such as slack:T0APRHSB676:D0APL3DF66S.