fix(#478): make EAP cert sources mutually exclusive

[VrtxOmega]

Fixes #478.

Summary

• represent each EAP certificate or private key with one typed EapCertSource::Path or EapCertSource::Blob value, making conflicting sources unrepresentable
• keep build_wifi_connection, wpa_eap, and wpa3_eap_192_bit infallible and preserve fluent builder chaining
• retain the existing path/blob constructors and setters; a later setter replaces the earlier source for that certificate or key
• update validation, NetworkManager serialization, docs, changelog, and regressions for both source variants

Open and WPA-PSK builder behavior is unchanged. The requested EapOptions field redesign is a breaking public-API change and is recorded in the changelog.

Validation

• cargo fmt --all -- --check
• cargo check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p nmrs --lib --all-features — 627 passed
• cargo test --doc --all-features --workspace — 129 passed, 16 ignored
• cargo test --all-features --workspace — 627 unit tests passed; 4 isolated-environment integration tests ignored as designed
• git diff --check origin/master...HEAD
• cargo semver-checks check-release -p nmrs — expected failure at the current 3.4.2 version because the enum redesign removes six public fields; the repository release policy classifies this as a major-version change

The source and behavior gates pass. The remaining release-policy choice is whether this redesign should target the next major release or use a compatibility migration; I have not added an unrequested version bump.

AI assistance

I used Codex to inspect the issue and maintainer review, implement the focused typed-source redesign and regression tests, and run the verification commands above. I reviewed the exact diff and verification output before updating this pull request.

[cachebag]

LGTM. Going to wait to merge. Want to think about the source break for a little ..

[cachebag]

@VrtxOmega wdyt?