enforce License.IsSeatCountEnforced if set (#31354)

* enforce License.IsSeatCountEnforced if set

If a license sets `IsSeatCountEnforced`, enforce the user limit therein
as a hard cap.

Fixes: https://mattermost.atlassian.net/browse/CLD-9260

* remove duplicate tests

* Improve user limit error messages and display

- Add separate error messages for licensed vs unlicensed servers
- Licensed servers: "Server exceeds maximum licensed users. ERROR_LICENSED_USERS_LIMITS"
- Unlicensed servers: "Server exceeds safe user limit. ERROR_SAFETY_LIMITS_EXCEEDED"
- Remove redundant "Contact administrator" text from activation errors shown to admins
- Fix system console to display actual server error messages instead of generic "Failed to activate user"

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Add license nil check and test coverage

- Add license != nil check in GetServerLimits to prevent panic
- Add test case to verify graceful handling of license being set to nil
- Ensures fallback to hard-coded limits when license becomes nil

Co-authored-by: lieut-data <lieut-data@users.noreply.github.com>

* Fix user limits tests to expect license-specific error IDs

Update test expectations to use the new license-specific error IDs:
- app.user.update_active.license_user_limit.exceeded for licensed server user activation
- api.user.create_user.license_user_limits.exceeded for licensed server user creation

Also update frontend to show actual server error messages instead of generic ones in system console.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Remove redundant license nil test

The test couldn't meaningfully verify nil license behavior since it relied on
hard-coded constants that can't be modified in the test.

Co-authored-by: lieut-data <lieut-data@users.noreply.github.com>

* Fix whitespace issue in limits_test.go

Remove unnecessary trailing newline to pass style checks.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* updated i18n

* s/ERROR_LICENSED_USERS_LIMITS/ERROR_LICENSED_USERS_LIMIT_EXCEEDED/, expand warning log

* Add 5% grace period for licensed user limits

- Add calculateGraceLimit() function with 5% or +1 minimum grace
- Apply grace period only to licensed servers with seat count enforcement
- Handle zero user licenses by returning zero grace limit
- Add comprehensive test coverage for grace period scenarios
- Unlicensed servers maintain existing hard-coded limits without grace

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix TestCreateUserOrGuestSeatCountEnforcement to account for 5% grace period

The test was failing because it expected user creation to fail at exactly
the license limit, but the implementation now includes a 5% grace period
before enforcement kicks in.

Changes:
- Update test cases to create users up to the grace limit (6 for a 5-user license)
- Add comments explaining the grace period calculation
- Both regular user and guest user creation tests now properly validate
  enforcement at the grace limit rather than the base license limit

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix TestUpdateActiveWithUserLimits to account for 5% grace period

Update test expectations to match the new grace period behavior:
- At base limit (100) but below grace limit (105): should succeed
- At grace limit (105): should fail
- Above grace limit (106): should fail

This aligns the tests with the license enforcement implementation
that includes a 5% grace period above the licensed user count.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: lieut-data <lieut-data@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/license.go

@@ -57,17 +57,18 @@ type LicenseRecord struct {
 }
 
 type License struct {
-	Id           string    `json:"id"`
-	IssuedAt     int64     `json:"issued_at"`
-	StartsAt     int64     `json:"starts_at"`
-	ExpiresAt    int64     `json:"expires_at"`
-	Customer     *Customer `json:"customer"`
-	Features     *Features `json:"features"`
-	SkuName      string    `json:"sku_name"`
-	SkuShortName string    `json:"sku_short_name"`
-	IsTrial      bool      `json:"is_trial"`
-	IsGovSku     bool      `json:"is_gov_sku"`
-	SignupJWT    *string   `json:"signup_jwt"`
+	Id                  string    `json:"id"`
+	IssuedAt            int64     `json:"issued_at"`
+	StartsAt            int64     `json:"starts_at"`
+	ExpiresAt           int64     `json:"expires_at"`
+	Customer            *Customer `json:"customer"`
+	Features            *Features `json:"features"`
+	SkuName             string    `json:"sku_name"`
+	SkuShortName        string    `json:"sku_short_name"`
+	IsTrial             bool      `json:"is_trial"`
+	IsGovSku            bool      `json:"is_gov_sku"`
+	IsSeatCountEnforced bool      `json:"is_seat_count_enforced"`
+	SignupJWT           *string   `json:"signup_jwt"`
 }
 
 type Customer struct {