fix: apply every chained exclude() call instead of only the last one

[moneebullah25]

Fixes #1816

Problem

QuerySet.exclude() replaced the previously collected exclude clauses, so qs.exclude(a=1).exclude(b=2) only applied the last call and silently returned rows that should have been excluded (this also affects update() and delete() on such querysets).

Change

• ormar/queryset/queryset.py: each exclude() call now appends its own group to exclude_clauses instead of replacing them. An argument-less exclude() is a no-op.
• ormar/queryset/queries/filter_query.py: every exclude clause is negated individually and the results are ANDed, so chained calls compose as NOT a AND NOT b. A single exclude(a=1, b=2) call keeps its existing meaning.
• tests/test_queries/test_chained_exclude.py: new tests for chained excludes, mixing with filter(), update()/delete(), empty exclude(), and or_/and_ groups with limit.

Checks run

• ruff format --check and ruff check on the changed files: pass
• mypy ormar tests --ignore-missing-imports: success
• DATABASE_URL=sqlite:///test.db pytest --cov=ormar --cov=tests tests/: 669 passed, 25 skipped, 0 failed, 100% coverage
• Added tests for the new behaviour
• Docs not updated (no documented behaviour changed)
• Not run against postgres/mysql backends (sqlite only)