chore(deps): Batch dependency PRs, clear all security advisories, unblock CI

[hoangsnowy]

Change description

Batches the mergeable dependency PRs into one branch and clears every open security advisory (NuGet + npm). Also unblocks CI, which was red on main regardless of the PR under test.

Cherry-picked (unchanged) from the open Dependabot PRs:

PR	What	Why
#131	npm_and_yarn group in tools/agentos-vscode — brace-expansion 5.0.6 → 5.0.8, linkify-it 5.0.1 → 5.0.2	Closes both open Dependabot alerts (#11, #12, both high)
#129	nuget-minor-patch group, 31 packages (10.0.9 → 10.0.10 across ASP.NET Core / EF Core / Extensions, OTel 1.17.0, MAF 1.13.0, …)	Also fixes the CI failure: Microsoft.AspNetCore.DataProtection.EntityFrameworkCore 10.0.9 pulled System.Security.Cryptography.Xml 10.0.9, which now carries 5 high-severity advisories → NU1903 as error → dotnet restore failed on the whole solution. The 10.0.10 bump resolves the patched transitive
#128	actions/setup-dotnet 5 → 6 in CI + azd-deploy workflows	Routine

Additional work in this PR:

• NSubstitute 5.3.0 → 6.0.0 (supersedes chore(deps): Bump NSubstitute from 5.3.0 to 6.0.0 #127, which was red). v6 annotates its public API for nullable reference types: Arg.Is<T> predicates now take T?, CallInfo.Arg<T>() / this[int] return nullable, and ReturnsNull<T> constrains T : class so it no longer binds against a string? / WorkspaceEntity? member. Under TreatWarningsAsErrors that was 28 CS86xx build errors. Fix is test-only and mechanical — null-forgiving operators at the substitute boundary, and the two ReturnsNull() calls swapped for an explicit Returns((T?)null).
• npm audit fix on tools/agentos-vscode — fast-uri (GHSA-v2hh-gcrm-f6hx, GHSA-4c8g-83qw-93j6 host confusion) and js-yaml (GHSA-52cp-r559-cp3m quadratic CPU on merge-key chains), both high, both transitive dev-only. Not yet surfaced as Dependabot alerts but flagged by npm audit.

Deliberately excluded: #130 (Microsoft.OpenApi 2.9.0 → 3.9.0). The ASP.NET Core OpenAPI XML-comment source generator shipped with Microsoft.AspNetCore.OpenApi 10.0.x assigns IOpenApiMediaType.Example, which became read-only in Microsoft.OpenApi 3.x → CS0200 in generated code, unfixable from this repo. The transitive floor from GHSA-v5pm-xwqc-g5wc stays satisfied — #129 already raises the pin to 2.11.0 (> 2.7.5). #130 should be closed / the major ignored until ASP.NET Core moves to OpenApi 3.

Type of change

• New feature (new agent / endpoint / capability)
• Bugfix
• Refactor (no behavior change)
• Documentation / README
• Infrastructure / CI configuration

Related issues

Closes Dependabot alerts #11 (brace-expansion) and #12 (linkify-it).
Supersedes #131, #129, #128, #127.

Checklist

• dotnet build AgentOs.slnx --configuration Release passes locally — 0 warnings, 0 errors
• dotnet test AgentOs.slnx --configuration Release passes — 832 passed, 0 failed, 14 skipped (+ 29 E2E skipped, RUN_AGENTOS_E2E unset)
• dotnet list package --vulnerable --include-transitive → no vulnerable packages
• npm audit in tools/agentos-vscode → 0 vulnerabilities; npm run build still bundles dist/extension.js
• No secrets committed
• README / docs updated if public behavior changed — n/a, dependency-only
• Measurable results provided when related to agent output quality — n/a

No UI surface touched, so no design-system round-trip or desktop screenshot applies.