refactor: Use typed interfaces for generating EnvFilters from config

[RasmusRendal]

I think this is just nicer than returning a string that the consumer is
expected to parse.

I wish we could do this without any string parsing at all, but this
functionality is still missing.

This is arguably not a super important PR, but it's just something weird I spotted and wanted to fix.

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Note

Low Risk
Internal refactor of filter construction with no change to telemetry export or security-sensitive paths; misconfiguration still surfaces as the same parse errors at init.

Overview
Refactors how tracing log levels from OpenTelemetry config become subscriber filters. ProviderConfig and StdoutLogsConfig no longer build a comma-separated filter string for callers to parse; get_filter now returns a Result<EnvFilter, ParseError> built via shared filter_from_config (general level, per-dependency overrides, main crate level).

In init_otel, stdout, OTLP logs, traces, and metrics layers call get_filter directly instead of EnvFilter::from_str, with parse failures still mapped through OtelInitError::EnvFilterError. Behavior should match the previous string format; the change is API shape and where parsing happens.

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[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.53%. Comparing base (af0a8ba) to head (93eac0f).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/config.rs	95.23%	1 Missing ⚠️
src/lib.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   75.83%   76.53%   +0.69%     
==========================================
  Files           5        5              
  Lines         269      277       +8     
==========================================
+ Hits          204      212       +8     
  Misses         65       65              

Files with missing lines	Coverage Δ
src/config.rs	70.76% <95.23%> (+4.10%)	⬆️
src/lib.rs	88.29% <75.00%> (ø)

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[mzaniolo]

LGTM