fix: prevent mixed =/- chars in Setext-style headings (+tests)

[Jah-yee]

Summary

This PR fixes issue #1604 - mixed =/- characters in Setext-style headers were incorrectly matched.

Changes

• markdown/blockprocessors.py: Changed regex from [=-]+ to (?:[=]+|[-]+) to match only homogeneous runs
• tests/test_syntax/blocks/test_headers.py: Added 3 test cases for mixed-char rejection

Tests Added

• test_setext_mixed_chars_not_h1: =- should not form an H1
• test_setext_mixed_chars_not_h2: -= should not form an H2
• test_setext_mixed_multiple_chars: Mixed runs are not valid headers

Closes #1604

[Jah-yee]

Added 3 test cases in tests/test_syntax/blocks/test_headers.py covering mixed-char rejection (test_setext_mixed_chars_not_h1, test_setext_mixed_chars_not_h2, test_setext_mixed_multiple_chars). All tests pass. The fix is ready for review!