chore: improve memory management for partially created mux

[Noroth]

WIP

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Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• If applicable, I have provided instructions for reviewers for manually validating my changes.
• Documentation has been updated on https://github.com/wundergraph/docs-website.
• I have read the Contributors Guide.

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[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-24e17f4d2478ed9c8354a0b50efcc76e151da1e3

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.40%. Comparing base (91268ec) to head (9e98e9e).

Files with missing lines	Patch %	Lines
router/core/graph_server.go	64.70%	3 Missing and 3 partials ⚠️
router/pkg/metric/cache_metrics.go	0.00%	1 Missing ⚠️
router/pkg/metric/engine_metrics.go	0.00%	1 Missing ⚠️
router/pkg/metric/otlp_metric_store.go	0.00%	1 Missing ⚠️
router/pkg/metric/prom_connection_metric_store.go	0.00%	1 Missing ⚠️
router/pkg/metric/router_runtime_metrics.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           ludwig/otel-fixes    #2995      +/-   ##
=====================================================
+ Coverage              61.20%   61.40%   +0.20%     
=====================================================
  Files                    259      259              
  Lines                  30039    30054      +15     
=====================================================
+ Hits                   18385    18456      +71     
+ Misses                 10158    10114      -44     
+ Partials                1496     1484      -12     

Files with missing lines	Coverage Δ
router/pkg/metric/cache_metrics.go	85.84% <0.00%> (ø)
router/pkg/metric/engine_metrics.go	75.90% <0.00%> (ø)
router/pkg/metric/otlp_metric_store.go	81.25% <0.00%> (ø)
router/pkg/metric/prom_connection_metric_store.go	83.33% <0.00%> (ø)
router/pkg/metric/router_runtime_metrics.go	76.38% <0.00%> (ø)
router/core/graph_server.go	85.26% <64.70%> (+0.15%)	⬆️

... and 13 files with indirect coverage changes

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