Add -Context and -Describe to BeforeEach and AfterEach (#1219)

[nohwnd]

Draft. Implements #1219.

BeforeEach and AfterEach only run around each test today. This exposes the block level variants that were scaffolded in the runtime but never had a public setter, using the switches I sketched in the issue:

Describe 'my tests' {
    BeforeEach -Context { <# create the resource once per Context #> }
    AfterEach  -Context { <# remove it after each Context #> }

    Context 'context 1' { It 'it 1' { } It 'it 2' { } }
    Context 'context 2' { It 'it 1' { } It 'it 2' { } }
}

• No switch keeps the classic per-test behavior (-It).
• The switches combine: BeforeEach -Context -It runs before the Context and before each test in it.
• -Describe targets Describe blocks, -Context targets Context blocks, using how the block was defined (FrameworkData.CommandUsed).
• They inherit top down like the other setups, a setup on an outer block runs before every matching nested block, and teardowns run in the opposite order.

No BeforeAll -Context, that one is redundant, BeforeAll already runs once and its side effects persist for the whole subtree. I dropped the now unused OneTimeBlock* leftovers while here.

Scoping: it runs on the block's existing OuterSetup/OuterTeardown, the same path BeforeEach/BeforeAll already use, so no extra scope is introduced, the setup is just inherited down to the child.

Verification

Full test.ps1 run passes (2617 passed, 0 failed). New tests in SetupTeardown.Tests.ps1 cover: -Context wrapping each Context and not the direct tests, inheritance into nested Contexts, -Describe targeting only Describe blocks, and the combined -Context -It order.

Still todo before this leaves draft

• docs on pester.dev
• decide if the failure message should name which of the inherited setups failed (cheap to add, each already runs on its own step)

[fflaten]

I think we'll quickly need a -Local/NoPropagate to only run for direct siblings. Nice to have for BeforeEach -It, but required for blocks as especially Context can have deep structures only used for organization or output.

A workaround would be to use nested Describe for that, but mixing child block types gets messy imo. (Personally I'd want Describe to be root-only and Context child-only groupings.)

[fflaten]

Should also work for root block? Currently blocked in Invoke-ContainerRun