Dependency/SDK updates; waiting on a Seq.Api 2026.1 build

[nblumhardt]

This PR updates Seq.Api to the 2026.1 dev version, aligns SDK and other dependencies to match the next Seq preview, adds the seqcli view * command group for metrics views, and updates retention policy and dashboarding commands to include support for the series object.

[nblumhardt]

Because DataSource is a value type (enum), this will fall back to Stream by default for older API versions that don't supply a data source explicitly (these only support stream anyway).

[nblumhardt]

A number of added files copied headers with incorrect copyright years. I've ripped the bandaid off and updated all of them so we stop propagating this mistake.

[KodrAus]

RUNNING SampleSetupTestCase                               
Use `--data-source` to specify `stream` or `series` as the retention target
FAIL
Xunit.Sdk.EqualException: Assert.Equal() Failure: Values differ
Expected: 0
Actual:   1
   at Xunit.Assert.Equal[T](T expected, T actual, IEqualityComparer`1 comparer) in /_/src/xunit.assert/Asserts/EqualityAsserts.cs:line 154
   at SeqCli.EndToEnd.RetentionPolicy.RetentionPolicyBasicsTestCase.ExecuteAsync(SeqConnection connection, ILogger logger, CliCommandRunner runner) in /home/runner/work/seqcli/seqcli/test/SeqCli.EndToEnd/RetentionPolicy/RetentionPolicyBasicsTestCase.cs:line 22
   at SeqCli.EndToEnd.Support.IsolatedTestCase.ExecuteTestCaseAsync() in /home/runner/work/seqcli/seqcli/test/SeqCli.EndToEnd/Support/IsolatedTestCase.cs:line 66
   at SeqCli.EndToEnd.Support.TestDriver.<>c__DisplayClass2_0.<<Run>b__1>d.MoveNext() in /home/runner/work/seqcli/seqcli/test/SeqCli.EndToEnd/Support/TestDriver.cs:line 53