Handle Targets Gracefully In Baseline And Ensemble Forecast Generation

[O957]

This PR:

• Adds supported_diseases constant; exported c("covid", "rsv", "flu") vector in utils.R, which replaces hardcoded c("covid", "rsv") checks.
• Adds get_unique_targets() function: reads target suffixes from hub time-series data for a given disease
• Adds get_target_label()function: maps target suffixes to human-readable labels.
• Updates generate_hub_baseline() with targets support: accepts optional targets parameter & defaults to all unique targets in time-series.
• Updates generate_hub_ensemble() with targets support (same as previous point)
• Updates composite actions for ensemble and baseline forecast production: they accept a targets input (JSON/YAML array).
• Updates disease supports to include influenza: see get_hub_name(), get_hub_repo_name(), and get_disease_name().

How targets work with the ensemble and baseline forecast generation:

• get_unique_targets() reads target suffixes from target-data/time-series.
• Both baseline and ensemble functions default to targets = NULL, which gets all unique targets .
• Both baseline and ensemble functions validate explicit targets against available targets, erroring if any are invalid.
• Composite baseline and ensemble actions expose targets as optional JSON array, defaults to all.

[sbidari]

It looks like some tests are failing. Can you update the failing tests?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 9.66%. Comparing base (e40f4e2) to head (75ed193).

Files with missing lines	Patch %	Lines
R/generate_hub_baselines.R	0.00%	25 Missing ⚠️
R/utils.R	0.00%	20 Missing ⚠️
R/generate_hub_ensemble.R	0.00%	15 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #137      +/-   ##
========================================
- Coverage   9.96%   9.66%   -0.31%     
========================================
  Files         12      12              
  Lines       1144    1180      +36     
========================================
  Hits         114     114              
- Misses      1030    1066      +36     

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[sbidari]

See comments regarding support for flu

[sbidari]

LGTM, thanks @O957