[Lee-Juntong]

Plain-text messages returned by _fmt_msg are cut off after 500 characters and have line breaks flattened. A multiline message longer than 500 characters therefore loses content in read/search/recent output.

Preserve the complete decoded content for ordinary text messages, including line breaks, blank lines, tabs and Unicode. Message count limits still apply, and non-text previews keep their existing behavior.

Validation: three synthetic regressions cover long Unicode text, a newline at the old cutoff, and whitespace preservation. All three fail on main and pass with this change. The remaining locally runnable tests pass.

Local validation uses Windows/Python 3.12 and synthetic fixtures. The consistency-checker file and two fingerprint-dependent existing tests were excluded by the local execution policy; no account databases, chat contents or keys are included.

Local branch suite: 84 passed, two tests deselected, consistency-checker file excluded.

[freanocank]

Thanks for the fix and regression tests. Adopted through #15 with your original commits preserved. The complete-text and whitespace-preservation regressions pass.

The combined suite passes all four macOS/Windows × Python 3.10/3.13 CI jobs; Windows reports 209 passed, and macOS reports 208 passed with the Windows-only test skipped.