fix(comms): remove limit on get_channels endpoint

[synoet]

removes the default limit

[whutchinson98]

in teo we trust

[claude]

Code review

Test Coverage Required

The PR changes database query behavior by making the limit parameter optional, but no tests validate the new limit: None behavior.

CLAUDE.md violation:

"Always run tests between changes that involve changes to db queries"

"When making changes to a db crate you should always update tests, and run prepare"

Source: rust/cloud-storage/CLAUDE.md

Issue details:

• The limit field changed from u32 to Option<u32> in models.rs
• SQL query binding changed from i64 to Option<i64> in dynamic.rs
• All existing tests in tests.rs use limit: Some(20) - none test the limit: None case

SQL behavior change:
When limit is None, PostgreSQL treats LIMIT NULL as "no limit" (returns all rows). This is a significant semantic change from the previous behavior where limits were always enforced (defaulted to 20, clamped between 20-100).

Recommendation:
Add a test case to validate the limit: None behavior. Example:

#[sqlx::test(
    migrator = "MACRO_DB_MIGRATIONS",
    fixtures(path = "../../../fixtures", scripts("channels"))
)]
async fn test_get_user_channels_dynamic_no_limit(pool: Pool<sqlx::Postgres>) {
    let user_id = MacroUserIdStr::parse_from_str("macro|user-1@test.com").unwrap();

    let params = GetChannelsRequest {
        macro_id: user_id.into_owned(),
        limit: None,  // Test the new behavior
        query: Query::Sort(SimpleSortMethod::UpdatedAt, None),
    }
    .into_params();

    let channels = get_user_channels_dynamic(&pool, &params).await.unwrap();

    // user-1 is a participant in 4 channels (A, B, C, D)
    assert_eq\!(channels.len(), 4, "Should return all channels when limit is None");
}