fix: stabilize thinking block rendering while streaming

[dnouri]

Summary

• Fix thinking block whitespace normalization to remove extra blank lines without stripping meaningful indentation.
• Avoid unnecessary buffer rewrites when incoming thinking deltas do not change rendered output.
• Ensure session resets clear all thinking-stream state (markers + raw buffer) to prevent stale state leaks.
• Refactor thinking-state cleanup naming/tests for clarity and lower duplication.

Testing

• === Byte-compile ===
  === Checkdoc ===
  OK
  === Package-lint ===
  === Unit Tests ===
  Making markdown-hide-markup buffer-local while locally let-bound!
  Ran 521 tests, 521 results as expected, 0 unexpected (2026-02-12 22:38:08+0100, 9.220074 sec)