fix(core/protocols): restore error fallback to UnknownError for protocols

[siddsriv]

Issue

Internal JS-6638

Description

restore error fallback for all protocols to "UnknownError" instead of "Unknown"
related fix: #7717

Testing

• local script to get error response and core package tests pass
• snapshot tests (diffs attached)

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts) or E2E tests.
  • It's not a feature.
• My E2E tests are resilient to concurrent i/o.
  • I didn't write any E2E tests.
• I added access level annotations e.g. @public, @internal tags and enabled doc generation on the package. Remember that access level annotations go below the description, not above.
  • I didn't add any public functions.
• Streams - how do they work?? My WebStream readers/locks are properly lifecycled. Node.js stream backpressure is handled. Error handling.
  • No streams here.

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[kuhe]

are there differences in the default error message for other protocols that were introduced in Nov '25?

[siddsriv]

yes, should we change all Unknowns to UnknownErrors?

[siddsriv]

i restored uniformity for this message for all protocols (smithy-client src here, what i think was the old behavior)

separate commits for each protocol, for ease of review