fix(llm_perplexity): align profile selector default with preconfig

[kgarg2468]

Description

nodes/src/nodes/llm_perplexity/services.json declares preconfig.default as sonar, but the perplexity.profile UI field defaults to sonar-pro. The generated README schema reflects sonar-pro, while the profile contract, hand-written README, and node test use sonar.

This creates two competing defaults for the same node. The selected model can depend on which configuration layer initializes the node, changing model behavior, token limits, and cost.

Steps to Reproduce

1. Open nodes/src/nodes/llm_perplexity/services.json.
2. Observe preconfig.default is sonar.
3. Observe fields["perplexity.profile"].default is sonar-pro.
4. Generate the node README or inspect the canvas schema default.
5. Observe that the generated field contract reports sonar-pro while the profile contract reports sonar.

Expected vs Actual Behavior

Expected: the preconfiguration default and profile-selector field default use the same profile. Evidence from the README contract and existing test configuration indicates the intended default is sonar.

Actual: preconfiguration and tests use sonar, while the UI field and generated schema use sonar-pro.

Severity

P2 - Medium. The node remains usable, but a newly configured node may select a different model than the documented default.

Platform

All platforms; the mismatch is in shared node metadata.

Acceptance Criteria

• preconfig.default and perplexity.profile.default both resolve to sonar.
• The generated README schema reports sonar.
• A deterministic regression test fails if a profile selector default diverges from preconfig.default.
• Perplexity node README validation and relevant unit/contract tests pass.

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