fix(init): allow long directory names

[gcomneno]

Summary

• validate snap names during init name resolution so directory-derived names can fall back to the default
• keep explicit invalid names failing with Snapcraft's snap name validation
• add coverage for long working directory names

Issue
Closes #6093

Validation

• pytest -q tests/unit/services/test_init.py tests/unit/commands/test_init.py -rs
• ruff check snapcraft/services/init.py tests/unit/services/test_init.py tests/unit/commands/test_init.py
• ruff format snapcraft/services/init.py tests/unit/services/test_init.py tests/unit/commands/test_init.py
• git diff --check

[gcomneno]

All functional checks are passing from my side. The remaining failures appear unrelated to this change:

• OSV-scanner is failing in the policy/security workflow.
• CI/publish (self-hosted, amd64) exits during the self-hosted runner pre-job authorization check with: "Author association: CONTRIBUTOR, is allowed: false".

Validation run locally:

• pytest -q tests/unit/services/test_init.py tests/unit/commands/test_init.py -rs
• ruff check snapcraft/services/init.py tests/unit/services/test_init.py tests/unit/commands/test_init.py
• ruff format snapcraft/services/init.py tests/unit/services/test_init.py tests/unit/commands/test_init.py
• git diff --check