chore: provide proxied configurations in update to "@slack/web-api@v8" and "@slack/webhook@v8" packages

[zimeg]

Summary

This pull request upgrades to the @slack/web-api and @slack/webhook v8 release candidates (slackapi/node-slack-sdk#2603), maintaining full proxy support by migrating from axios/https-proxy-agent to native fetch with the undici ProxyAgent.

Changes:

• Send API method requests with the @slack/web-api v8 WebClient, using its fetch option instead of the removed agent option
• Send webhook requests with the @slack/webhook v8 IncomingWebhook and WebhookTrigger clients, using their fetch option and retryConfig retry policies
• Extract a shared, side-effect-free proxiedFetch factory (src/proxies.js) that returns a custom fetch backed by an undici ProxyAgent dispatcher — injected into both the API and webhook clients so proxying stays opt-in and testable without any global dispatcher state
• Instrument both clients through the v8 addAppMetadata exports (rc.2 restores addAppMetadata, WebhookTrigger, and retryPolicies), so the SDK sets the User-Agent internally
• Update API error handling to use the v8 Error subclasses (WebAPIPlatformError, WebAPIHTTPError, etc.)

Dependencies removed: axios, axios-retry, https-proxy-agent
Dependencies added: undici (for ProxyAgent)
Dependencies upgraded: @slack/web-api ^7 → ^8.0.0-rc.2, @slack/webhook ^7 → ^8.0.0-rc.2, @slack/logger ^4 → ^5.0.0-rc.1

The proxy input and HTTPS_PROXY environment variable continue to work unchanged for end users.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[changeset-bot]

🦋 Changeset detected

Latest commit: ccbeecd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@slack/slack-github-action	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.86%. Comparing base (be88bdd) to head (ccbeecd).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #619      +/-   ##
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- Coverage   99.86%   99.86%   -0.01%     
==========================================
  Files           7        8       +1     
  Lines         730      725       -5     
==========================================
- Hits          729      724       -5     
  Misses          1        1              

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[zimeg]

🪄 A comment about webhook implementations while we explore changes-

[zimeg]

[DRAFT — do not submit yet] Why the undici-types override exists

@types/node pulls undici-types transitively at a version (7.18) that trails the undici we depend on directly (7.28). Their Dispatcher types are nominally distinct, so passing a ProxyAgent to globalThis.fetch failed to type check.

Ruled out the alternatives:

• Downgrade @types/node — every version pins undici-types ≤7.18; older pins even lower. Can't reach 7.28.
• Downgrade undici — would sacrifice runtime fixes to appease the type checker; also must be frozen forever.
• Add undici-types as a devDep — npm can't reconcile ~7.18 (from @types/node) with ^7.28, so it installs two copies; fetch's types still resolve the nested 7.18. No effect.

overrides is the only mechanism that rewrites the transitive pin, forcing the nested undici-types to 7.28 — verified byte-identical to runtime undici@7.28's bundled types. Lets the dispatcher field type check without an any cast.

Maintenance note: the override is a literal ^7.28.0, not self-following. If a future undici major outpaces it the mismatch returns — but CI's npm run check fails loudly, so it can't regress silently.