[#1069] Make validation error singular when only 1 character is entered

[stefanvi]

Closes #1069

DOD checklist

For PR maintainer

Perform these checks before marking the PR as ready:

• I have linked the PR to at least one issue.
• I assigned the PR to myself.
• I have added a description how to test this PR (see "Review Instructions").
• I have added documentation where necessary.

For reviewer

• I have read all code changes.
• I have audited the code quality.
• I have tested the changes either or both:
  • locally
  • on the test environment (preferred)
• I have validated that the PR is functionally correct (use-cases, figma designs, etc.)

Review instructions

Import a candidate list where one candidate has e.g. a BSN consisting of a single character. It shows the singular form

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
33.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Sigrid maintainability feedback

---

🔒Enable security insights in your CI pipeline to detect vulnerabilities early.
Start using Sigrid CI for Security today. Learn more.

---

⏸️️ You are still below your objective of 3.5 stars.

Show details

Sigrid compared your code against the baseline of 2026-08-27.

👍 What went well?

You fixed or improved 0 refactoring candidates.

👎 What could be better?

Unfortunately, 1 refactoring candidates were introduced or got worse.

Risk	System property	Location
🟠	Unit Size (Worsened)	src/form/validation_error.rs ValidationError.message(Locale)

📚 Remaining technical debt

0 refactoring candidates didn't get better or worse, but are still present in the code you touched.

View this system in Sigrid to explore your technical debt

⭐️ Sigrid ratings

System property	System on 2026-08-27	Before changes	New/changed code
Duplication	5.3	5.5	5.5
Unit Size	3.3	0.5	0.5
Unit Complexity	4.6	5.5	5.5
Unit Interfacing	2.5	5.5	5.5
Module Coupling	3.0	N/A	N/A
Component Independence	5.5	5.5	5.5
Component Entanglement	N/A	N/A	N/A
Maintainability	4.0	3.4	3.4

💬 Did you find this feedback helpful?

We would like to know your thoughts to make Sigrid better.
Your username will remain confidential throughout the process.

• ✅ Yes, these findings are useful
• 🔸 The findings are false positives
• 🔹 These findings are not important to me

---

View this system in Sigrid

[github-actions]

PDF diff summary

Template	Input	Status
model-h1	model-h1-example-1.pdf	🔵 identical
model-h1	model-h1-example-2.pdf	🔵 identical
model-h1	model-h1-example-3.pdf	🔵 identical
model-h3-1	model-h3-1-example-1.pdf	🔵 identical
model-h3-1	model-h3-1-example-2.pdf	🔵 identical
model-h3-1	model-h3-1-example-3.pdf	🔵 identical
model-h3-2	model-h3-2-example-1.pdf	🔵 identical
model-h3-2	model-h3-2-example-2.pdf	🔵 identical
model-h3-2	model-h3-2-example-3.pdf	🔵 identical
model-h4	model-h4-example-1.pdf	🔵 identical
model-h4	model-h4-example-2.pdf	🔵 identical
model-h4	model-h4-example-3.pdf	🔵 identical
model-h9	model-h9-example-1.pdf	🔵 identical
model-h9	model-h9-example-2.pdf	🔵 identical
model-h9	model-h9-example-3.pdf	🔵 identical
model-i1	model-i1-example-1.pdf	🔵 identical
model-i1	model-i1-example-2.pdf	🔵 identical
model-i4	model-i4-example-1.pdf	🔵 identical
model-i4	model-i4-example-2.pdf	🔵 identical