fix: handle slash branches in GitHub URLs and multi-dot file extensions

[chengyixu]

Summary

• Fixes GitHub blob URL parsing to support branch names containing slashes (e.g., feature/new-validation)
• Fixes file extension detection to correctly handle multi-dot filenames (e.g., my.asyncapi.yaml)

Fixes #1940
Part of microgrant #2125

Changes

1. GitHub URL branch parsing (src/domains/services/validation.service.ts)

The regex ([^\/]+) for branch names did not allow slashes, causing URLs like https://github.com/org/repo/blob/feature/new-validation/spec.yaml to fail with 404 errors.

Fix: Match everything after /blob/ as a single string, then generate candidate branch/path splits. Try each candidate progressively until one succeeds:

• Candidate 0: branch=main, path=src/file.yaml (most common case, works for simple branches)
• Candidate 1: branch=feature/new-validation, path=spec.yaml (works for slash branches)

The resolver's read() method now tries each candidate in sequence, stopping at the first successful fetch.

2. File extension detection (src/domains/models/SpecificationFile.ts, src/apps/cli/commands/new/file.ts)

The old code used name.split('.')[1] which returns the second segment after splitting by dots. For my.asyncapi.yaml, this returned asyncapi instead of yaml.

Fix: Use name.split('.')[parts.length - 1] to get the actual file extension (last segment after the final dot).

Test plan

• All existing unit tests pass (60/60)
• All existing integration tests pass (31/31 validate, 2/2 new file)
• Verified regex produces correct candidate URLs for slash branches
• Verified extension detection correctly identifies yaml in my.asyncapi.yaml

🤖 Generated with Claude Code

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Latest commit: 863f644

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[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[chengyixu]

Closing in favor of #2197 which has the cleaner splitBranchAndPath approach. The new/file.ts fix from this PR will be ported to #2197.