fix: improve join command servers handling

[RomanHotsiy]

What/Why/How?

This pull request resolves an issue with the join command where it would incorrectly merge OpenAPI documents with different servers configurations, leading to a semantically invalid result.

The updated logic now handles server merging based on whether the input files share the same server definitions.

Reference

Resolves https://github.com/Redocly/redocly-cli/issues/2118

Testing

Check yourself

• [ ] Code changed? - Tested with redoc (internal)
• [x] All new/updated code is covered with tests
• [ ] New package installed? - Tested in different environments (browser/node)

Security

• [x] Security impact of change has been considered
• [x] Code follows company security practices and guidelines

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 3251ad4f4aa9f9461a814ef7cadce68446e67b34

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@redocly/cli	Patch
@redocly/openapi-core	Patch
@redocly/respect-core	Patch

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[github-actions[bot]]

Command	Mean [s]	Min [s]	Max [s]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	1.466 ± 0.038	1.416	1.547	1.02 ± 0.03
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	1.441 ± 0.026	1.408	1.503	1.00