http2: initialize keepAliveTimeout and keepAliveTimeoutBuffer when allowHTTP1 is true

[amyssnippet]

This PR fixes an issue where keepAliveTimeout and related timeout properties were undefined on Http2SecureServer instances, even when allowHTTP1: true was set.

When falling back to HTTP/1.1, the server should respect standard HTTP timeout behaviors. This change ensures these properties are initialized with their default values (matching http.Server and https.Server), preventing inconsistent behavior during HTTP/1.1 fallback.

Fixes: #59783

[nodejs-github-bot]

Review requested:

• @nodejs/http2
• @nodejs/net

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.74%. Comparing base (3ab9dd8) to head (0efefc1).

Files with missing lines	Patch %	Lines
lib/internal/http2/core.js	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61713      +/-   ##
==========================================
+ Coverage   89.73%   89.74%   +0.01%     
==========================================
  Files         675      675              
  Lines      204502   204517      +15     
  Branches    39304    39302       -2     
==========================================
+ Hits       183502   183551      +49     
+ Misses      13283    13252      -31     
+ Partials     7717     7714       -3     

Files with missing lines	Coverage Δ
lib/internal/http2/core.js	95.13% <73.33%> (-0.09%)	⬇️

... and 38 files with indirect coverage changes

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[amyssnippet]

@ronag the PR is ready for review.
the failed macos test is flaky unrelated to my change, my changes are isolated to lib/internal/http2/core.js and test/parallel/test-http2-https-fallback-http-server-options.js, and the error comes out to be debugger related tests which is not introduced by this change.

[amyssnippet]

@ronag kindly rerun the failing check and also run the internal jenkins ci so that changes are verified to merge

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71226/

[amyssnippet]

@ronag is everything good?? all checks successfull

kindly review, if any changes then let me know