Make ApplicationContext inaccessible in ServerWebExchange

[JohnNiang]

What type of PR is this?

/kind improvement /area core /area plugin /milestone 2.20.x

What this PR does / why we need it:

Plugins can implement their own RouterFunctions and ControllerMappings, but those might expose root ApplicationContext for plugins, which is not expected.

So this PR fixes the insecure access to root ApplicationContext.

Does this PR introduce a user-facing change?

None

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Project coverage is 58.13%. Comparing base (6cd8dc8) to head (fd2a4fb). Report is 80 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/java/run/halo/app/infra/SecureServerRequest.java	66.66%	1 Missing :warning:
...va/run/halo/app/infra/SecureServerWebExchange.java	66.66%	1 Missing :warning:
.../run/halo/app/plugin/AggregatedRouterFunction.java	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6679 +/- ## ============================================ - Coverage 58.18% 58.13% -0.05% - Complexity 3774 3945 +171 ============================================ Files 651 678 +27 Lines 22125 23127 +1002 Branches 1538 1578 +40 ============================================ + Hits 12873 13446 +573 - Misses 8641 9061 +420 - Partials 611 620 +9 ```

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[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing

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