chore: declare framework deps as optional in adapter packages

**Depends on #354 (monorepo migration) and #355 (dependency update).**

After the monorepo split, each adapter package still declares its framework as a hard dependency. A user installing `hedera-agent-kit-langchain` into a project that already has `langchain` will get a second copy if our pinned version does not intersect with theirs — causing type-identity mismatches and duplicate installs.

JS reference: [PR #973](https://github.com/hashgraph/hedera-agent-kit-js/pull/973) (`fix(deps): avoid duplicate framework copies via peer deps`)

## Scope

- `packages/langchain/pyproject.toml` — move `langchain`, `langchain-openai`, `langchain-anthropic`, `langchain-groq`, `langchain-mcp-adapters` to `[project.optional-dependencies]` (uv extras)
- `packages/adk/pyproject.toml` — move `google-adk` to optional
- `packages/mcp/pyproject.toml` — move `mcp` to optional
- Update install instructions in README and examples (e.g. `pip install hedera-agent-kit-langchain[langchain]`)

Start by inspecting the dependency tables in packages/langchain/pyproject.toml, packages/adk/pyproject.toml, and packages/mcp/pyproject.toml, using PR #973 as the packaging reference. Update the README and examples with the corresponding extras install commands, then verify that each adapter exposes its framework through optional dependencies without retaining a hard dependency.

Written by the indexing model from the issue text.