feat: improve graph building and modernize tooling

[JBBianchi]

Many thanks for submitting your Pull Request ❤️!

What this PR does / why we need it:

This PR combines two related improvement tracks:

1. Graph representation and Mermaid generation improvements
2. Tooling, packaging, and browser-consumption modernization

The graph-related changes improve how workflow graphs are built, simplified, and rendered, especially for nested flows and Mermaid output.
The modernization changes refresh the project’s build/test/release toolchain, strengthen package validation, and make browser usage clearer across both ESM and UMD scenarios.

Overall, this PR is meant to make the SDK easier to consume, easier to maintain, and more predictable both at runtime and during release validation.

Graph representation and Mermaid generation improvements:

This part of the PR improves the workflow graph model itself as well as the Mermaid generation pipeline that consumes it.

Main functional changes:

• adds support for building a flattened/simplified graph
• supports removing internal subgraph entry/exit port nodes and remapping edges accordingly
• fixes nested-flow transition handling so the last node in a subgraph exits that subgraph instead of incorrectly ending the full workflow
• fixes explicit end transitions so they resolve to the root end node
• removes duplicate nodes and duplicate edges during graph construction
• merges duplicate transitions that resolve to the same target
• allows empty task lists / empty workflows to produce a valid minimal graph instead of failing

Graph API/model changes:

• buildGraph now supports flattened / port-free graph generation through additional options
• flattened graph types/helpers were introduced
• graph edges now use targetId instead of destinationId
• flattened nodes expose parentId instead of carrying full parent graph references
• graph nodes/subgraphs now keep a reference to the related workflow task

Mermaid-specific changes:

• Mermaid generation now works from the simplified graph representation
• nested diagrams are cleaner because they no longer expose internal routing nodes/edges unnecessarily
• the hidden-node styling was adjusted to work around a Mermaid rendering issue

This part of the PR is covered by graph and Mermaid tests, including simplified graph scenarios, empty workflows, and duplicate-switch-transition handling.

Tooling, packaging, and browser-consumption modernization:

This part of the PR modernizes the project toolchain and release pipeline.

Build/test/lint modernization:

• migrates from the previous Rollup/Jest/ESLint/Prettier setup to tsdown, Vitest, and Oxc
• updates TypeScript/compiler baselines and standardizes LF line endings
• separates tool/test typechecking more cleanly
• refreshes local scripts and contributor/release documentation

Packaging and publish validation:

• updates the published package layout and metadata for clearer modern consumption
• exposes explicit CommonJS, native ESM, and UMD outputs
• updates exports and type declaration wiring for import/require consumers
• adds stronger publish validation, including package smoke tests and package quality checks before release
• tightens CI and npm release workflows so linting, codegen verification, tests, packaging, and publish validation are part of the pipeline

Browser-consumption changes:

• updates browser examples to use native ESM as the primary model
• keeps UMD/global examples available for standalone browser usage
• clarifies README/examples around how to consume the SDK in browser environments

Compatibility/runtime notes:

• the repository is now aligned with Node.js 22+
• packaging/output file layout changed as part of the modernization work
• this is primarily a tooling/distribution modernization, not a change to workflow semantics

Special notes for reviewers:

This PR mixes a feature-level improvement area (graph / Mermaid behavior) with a broad infrastructure modernization track.
They are mostly orthogonal, but they landed together because the modernization work also affects how the package is built, tested, validated, and demonstrated in browser examples.

Suggested review split:

• first review the graph / Mermaid behavior changes
• then review the tooling / packaging / CI / browser example modernization

Additional information (if needed):

Validation coverage includes:

• graph builder tests
• simplified graph tests
• Mermaid output tests
• plain-object workflow typing checks
• package/build/publish validation through the updated toolchain

[lornakelly]

@JBBianchi Thanks for the PR, would you please update the PR description with the changes made to improve graph building etc as well

[ricardozanini]

Great work updating all this! I just have a few comments regarding the pipeline updates.

[JBBianchi]

@JBBianchi Thanks for the PR, would you please update the PR description with the changes made to improve graph building etc as well

@lornakelly Updated the PR description with a more comprehensive summary, including the graph building improvements.

[cdavernas]

LGTM! Cheers ❤️

[lornakelly]

LGTM, thanks