deps: consolidate six Dependabot bumps and group future updates

[ivanball]

Replaces six of the seven open Dependabot PRs with one, and adds grouping config so this stops recurring.

What ships

Package	From	To	Was
actions/cache	4	6	#146
Azure.Monitor.OpenTelemetry.AspNetCore	1.5.0	1.6.0	#148
AngleSharp	1.5.2	1.6.0	#149
bunit	2.7.2	2.8.6	#150
SQLitePCLRaw.bundle_e_sqlite3	3.0.4	3.0.5	#153
System.IdentityModel.Tokens.Jwt	8.19.2	8.21.0	#154

Built by regeneration, not by merging the branches. Merging them would have conflicted: they touch overlapping packages.lock.json files (AngleSharp and bunit hit the same two; the analyzers PR touches all 26), and every merge re-BEHINDs the rest under "require branches to be up to date". Applying the version lines and running one dotnet restore --force-evaluate sidesteps that, and costs one CI run instead of six.

Deliberately NOT included: #147 (analyzers)

SonarAnalyzer.CSharp 10.29 → 10.30 ships two new rules, S8969 and S8970, which fail this build with 41 errors across 11 files under TreatWarningsAsErrors:

76 error S8969  (redundant null-forgiving operator)
 6 error S8970  (null-forgiving where nullable warnings are disabled)

The fixes are mechanical (drop a redundant !), but that is analyzer remediation, not a dependency bump, and bundling 41 unrelated source edits into a version-bump PR would make both harder to review and to revert. #147 stays open for its own PR.

Grouping config, so this stops recurring

dependabot.yml already grouped analyzers and opentelemetry — which is why two of the seven arrived pre-grouped. Two more groups close the gap:

• testing — keeps bUnit and its AngleSharp dependency in one PR instead of two that touch the same lock files.
• minor-and-patch — catch-all so ungrouped minor/patch updates arrive as one weekly PR. Majors stay ungrouped: those deserve their own read.

This matters because of what a Common PR costs: 13 required checks including a MAUI windows build, three browser a11y legs, a BenchmarkDotNet gate and a Testcontainers Redis tier.

Verification

• Full solution Release: 2483/2483 green, 0 warnings, 0 errors.
• 15 lock files regenerated.

Not a deploy: Common merges are not releases. These reach ADC/Store at the next version sweep.