chore: rename String.ValidPos to String.Pos

[TwoFX]

This PR renames String.ValidPos to String.Pos, String.endValidPos to String.endPos and String.startValidPos to String.startPos.

Accordingly, the deprecations of String.Pos to String.Pos.Raw and String.endPos to String.rawEndPos are removed early, after an abbreviated deprecation cycle of two releases.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 4296f8deeece4fdf6dbe7d5d6b5e23bd6a250d13 --onto 5a4226f2bdcc6299df76285b1d30f238546c09fe. You can force Mathlib CI using the force-mathlib-ci label. (2025-11-18 13:08:23)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 9fc90488ce239cfe8e4317269d37fe4160787d0d --onto 5a4226f2bdcc6299df76285b1d30f238546c09fe. You can force Mathlib CI using the force-mathlib-ci label. (2025-11-19 10:36:20)
• 💥 Mathlib branch lean-pr-testing-11240 build failed against this PR. (2025-11-24 15:40:55) View Log
• ✅ Mathlib branch lean-pr-testing-11240 has successfully built against this PR. (2025-11-24 18:18:54) View Log

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 4296f8deeece4fdf6dbe7d5d6b5e23bd6a250d13 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-11-18 13:08:24)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 9fc90488ce239cfe8e4317269d37fe4160787d0d --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-11-19 10:36:22)
• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2025-11-24 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2025-11-24 15:36:03)