introduce MailClientProvider for dynamic mail client generation

[vzickner]

Check List:

• Unit tests: YES
• Documentation: NO

[filiphr]

Thanks @vzickner. Looks good, I think we can do some small cleanups and maybe deprecate some of the old methods.

I've left some comments, the same comments are also valid for the BPMN counterparts.

Some other suggestions from a Claude Code Review:

• BPMN EmailTestCase.reinitilizeMailClients() should reset mailClientProvider

  Unlike the CMMN CmmnEmailTestCase.reinitializeMailClients() which calls setMailClientProvider(null), the BPMN version does not. Works by coincidence currently but fragile.

• CmmnEmailTestCase.tearDown() restores provider then immediately overwrites it

  tearDown() calls setMailClientProvider(initialMailClientProvider) followed by reinitializeMailClients() which calls setMailClientProvider(null). The restoration is dead code.

[filiphr]

I would use StringUtils.isNotBlank(tenantId) here. Reason is the fact that the empty tenant is null with Oracle, but empty string with other DBs.