Add support for 4.1 and 4.5 models

[kuzminT]

• Adds support for new OpenAI model variants: gpt-4.1 and gpt-4.5. These models currently cause errors when used with this library.
• Related to the upstream PR in the original repository.

[Copilot]

Pull Request Overview

Adds support for the new gpt-4.1 and gpt-4.5 model variants by mapping them to the existing o200k_base encoding and updating the README to reflect those additions.

• Introduce exact and prefix-based mappings for gpt-4.1 and gpt-4.5 in encoding.go
• Update documentation tables in README.md to list the new models under available encodings and models

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
encoding.go	Added "gpt-4.1" and "gpt-4.5" entries to both MODEL_TO_ENCODING and MODEL_PREFIX_TO_ENCODING maps
README.md	Expanded tables and prose to include gpt-4.1 and gpt-4.5 in the available encodings/models sections

Comments suppressed due to low confidence (3)

encoding.go:25

• Add or update unit tests to cover the new mappings for gpt-4.5 and gpt-4.1 so that encoding lookups for these models are validated.

	"gpt-4.5":       MODEL_O200K_BASE,

encoding.go:72

• The inline comment example still references gpt-4o-.... Update it to gpt-4.5-... to match this mapping.

	"gpt-4.5-":       MODEL_O200K_BASE,  // e.g., gpt-4o-2024-05-13, etc.

encoding.go:73

• The inline comment example still references gpt-4o-.... Update it to gpt-4.1-... to reflect this prefix mapping correctly.

	"gpt-4.1-":       MODEL_O200K_BASE,  // e.g., gpt-4o-2024-05-13, etc.