fix: throw errors for invalid duration tokens in Gantt diagrams

[gsha22]

📑 Summary

These fixes ensure that if there are extra characters after a task duration, an error is thrown instead of failing silently.

We also added a test to ensure that it throws errors on invalid duration tokens.

Resolves #6586

In association with the following doc pr: #7212

📏 Design Decisions

Added conditional logic in mermaid/src/diagrams/gantt/ganttDb.js on line 394 to ensure an error is thrown if check fails.

Our comment:

// If the string is not a valid ISO date (handled earlier) and it is
// also not a recognised duration token, that's an invalid duration
// token. Throw an error so the renderer fails loudly and the user
// gets a clear message instead of silently omitting the task.
// Example invalid tokens: `24de`, `24d+`, `24d=`, `24d7`.

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Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 3.58%. Comparing base (39d070f) to head (ae19587).

Files with missing lines	Patch %	Lines
packages/mermaid/src/diagrams/gantt/ganttDb.js	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #7227      +/-   ##
==========================================
+ Coverage     3.56%   3.58%   +0.01%     
==========================================
  Files          473     473              
  Lines        47566   47577      +11     
  Branches       734     738       +4     
==========================================
+ Hits          1696    1705       +9     
- Misses       45870   45872       +2     

Flag	Coverage Δ
unit	3.58% <84.61%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
packages/mermaid/src/diagrams/gantt/ganttDb.js	77.13% <84.61%> (+0.06%)	⬆️

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Build	Status	Details	Updated (UTC)
default (Inspect)	⚠️ Changes detected (Review)	4 changed	Dec 11, 2025, 6:29 AM

[omkarht]

Implementation is excellent, but test coverage is insufficient, please add tests for all invalid patterns mentioned in your comments plus regression tests for valid duration and edge cases (currently 1 test, need 5+).

[gsha22]

Implementation is excellent, but test coverage is insufficient, please add tests for all invalid patterns mentioned in your comments plus regression tests for valid duration and edge cases (currently 1 test, need 5+).

Thank you! Added some more pattern tests, as well as some regression tests for sanity check. Let me know if you need anything else!