fix: preserve keyword placeholder strings for all email provider credentials

[ankita10119]

🔧 Changes

When AUTH0_PRESERVE_KEYWORDS is enabled and a local config file uses a keyword placeholder string for credentials (e.g. "@@SMTP_CREDENTIALS@@"), emailProviderDefaults now preserves it as-is instead of attempting to apply masking logic.

Previously, for smtp, the in operator was used directly on the placeholder string, causing a hard TypeError crash during export. For all other providers (ses, mailgun, mandrill, sendgrid, sparkpost, azure_cs, ms365), the string was silently spread into the credentials object as character-indexed keys (e.g. {"0": "@", "1": "@", ...}), which caused Auth0 API to reject the payload with a 400 on subsequent deploy.

The fix adds a typeof credentials !== 'string' guard to each provider block in emailProviderDefaults. This is purely additive — all existing behavior for object and undefined credentials is unchanged.

📚 References

Fixes #1467

🔬 Testing

Unit tests added for all affected providers (smtp, ses, mailgun, azure_cs, ms365) covering the keyword placeholder string case.

Manual end-to-end testing performed:

• Confirmed the TypeError crash during export with AUTH0_PRESERVE_KEYWORDS=true and credentials: "@@SMTP_CREDENTIALS@@" in local config
• Confirmed export succeeds after the fix and the placeholder is preserved in the output file
• Confirmed Auth0 API returns 400 Additional properties not allowed when garbage character-indexed keys are deployed (validating the severity of the other providers' issue)

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.52%. Comparing base (a00a055) to head (d7f2f5d).

Files with missing lines	Patch %	Lines
src/context/defaults.ts	64.70%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1470      +/-   ##
==========================================
+ Coverage   80.51%   80.52%   +0.01%     
==========================================
  Files         163      163              
  Lines        7745     7750       +5     
  Branches     1721     1726       +5     
==========================================
+ Hits         6236     6241       +5     
  Misses        808      808              
  Partials      701      701              

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