fix(client/tls): correct PEM label and add Ed25519 key support

[fredbi]

TLSClientAuth marshaled RSA keys as PKCS#1 and ECDSA keys as SEC1, then labeled both PEM blocks as "PRIVATE KEY" — a label conventionally reserved for PKCS#8. The output round-tripped through tls.X509KeyPair only because Go's parser is lenient; stricter consumers would reject it.

Switch to a single x509.MarshalPKCS8PrivateKey call, which matches the "PRIVATE KEY" label naturally and transparently supports Ed25519 (previously rejected by the type switch's default branch).

Also fix gencerts_test.go: writePKCS1KeyPair wrote PKCS#1 RSA bytes under an "EC PRIVATE KEY" label — same class of latent mislabel.

Adds an Ed25519 subtest under TestRuntimeTLSOptions.

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.06%. Comparing base (1c5e373) to head (d62bea7).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
client/tls.go	33.33%	1 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (33.33%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
+ Coverage   85.04%   85.06%   +0.02%     
==========================================
  Files          60       60              
  Lines        3978     3970       -8     
==========================================
- Hits         3383     3377       -6     
+ Misses        451      449       -2     
  Partials      144      144              

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