fix: preserve booking method quoted string in open directive formatting

[polarmutex]

Summary

Fixes #836.

The formatter was stripping the leading " from booking method strings on open directives. For example:

2024-01-01 open Assets:Investment:Retirement:Fidelity-Roth-IRA:FXAIX FXAIX "FIFO"

was being mangled to:

2024-01-01 open Assets:Investment:Retirement:Fidelity-Roth-IRA:FXAIX FXAIX FIFO"

Root cause: In generate_template_edits, the code used rest_content.find(char::is_alphabetic) to skip leading whitespace/non-alpha chars before a currency token. When the rest content was a quoted booking method like "FIFO", it found F at index 1 (skipping the "), producing the corrupted output.

Fix: Check whether rest_content starts with " before applying the alpha-search. If it does, it's a quoted token (booking method) and should be preserved verbatim with a single leading space.

Test plan

• Added test_open_directive_booking_method_preserved — verifies that "FIFO" booking methods on open directives are preserved intact after formatting
• Verifies idempotency: second format pass produces no edits
• All 54 existing formatting tests continue to pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.91%. Comparing base (d0d7e0c) to head (2d63ea6).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #848      +/-   ##
==========================================
+ Coverage   80.88%   80.91%   +0.03%     
==========================================
  Files          31       31              
  Lines       12502    12522      +20     
==========================================
+ Hits        10112    10132      +20     
  Misses       2390     2390              

Files with missing lines	Coverage Δ
crates/lsp/src/providers/formatting.rs	92.26% <100.00%> (+0.10%)	⬆️

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