fix(core): count all messages omitted by head-tail context

[Tivonsico]

Why are these changes needed?

Head/tail context windows discard incomplete boundary tool messages, but the omission placeholder still counts configured window sizes. Count the actual retained messages so an eight-message history retaining two messages says Skipped 6 messages. Correct the method docstring's reversed oldest/recent wording.

Related issue number

Fixes #8328

Checks

• Added seven offline regression cases covering four boundary-window combinations, empty/short contexts, and preservation of stored history; three fail against unchanged 0.7.5 and all pass with this fix.
• The new regressions and the existing model-context test suite pass: 16 tests.
• Ruff formatting/lint and focused Pyright/Mypy checks passed; git diff --check passed.
• Full repository checks have not been run.

AI assistance was used to investigate, implement, and verify this change.

[Tivonsico]

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