fix(config): graceful failure fallback to default

[duncanista]

What does this PR do?

Adds per-field graceful deserialization to the datadog-agent-config crate. Previously, if a single config field had the wrong type (e.g., DD_FLUSH_TIMEOUT=abc or flush_timeout: [1,2,3] in YAML), the entire config
source would fail and fall back to Config::default() — losing every valid field alongside the bad one. Now, only the invalid field falls back to its default while all other correctly-typed fields are preserved.

The fix introduces one generic deserializer function, deserialize_with_default<T: Deserialize + Default>, which catches serde errors at the field level and returns T::default(). This is applied via
#[serde(deserialize_with)] to all previously-unprotected fields across EnvConfig, YamlConfig, and their nested structs. Several existing deserializers (deserialize_optional_duration_from_seconds,
deserialize_optional_duration_from_microseconds, deserialize_trace_propagation_style, deserialize_key_value_pair_array_to_hashmap, deserialize_service_mapping, deserialize_apm_replace_rules) were also hardened to
catch errors instead of propagating them.

Motivation

In production, a single misconfigured environment variable or YAML field should not wipe out the entire agent configuration. The agent should log a warning for the bad field and continue with defaults for that
field only, while respecting all other valid configuration.

Additional Notes

• No structural changes to EnvConfig, YamlConfig, nested structs, Config, merge_config, or ConfigSource::load. The change is purely additive #[serde(deserialize_with)] annotations and error-catching in existing
  deserializer functions.
• All existing deserializers remain in use — no dead code introduced.
• The test_parse_duration_from_microseconds test was updated: negative and float values for microsecond durations now gracefully return None instead of erroring, consistent with the graceful-failure goal.

Describe how to test/QA your changes

Two comprehensive regression tests were added:

• test_all_yaml_fields_wrong_type_fallback_to_default — Creates a YAML file where every field (including deeply nested OTLP config) is set to the wrong type ([1, 2, 3]). Asserts load() succeeds and config ==
  Config::default().
• test_all_env_fields_wrong_type_fallback_to_default — Sets every non-string env var to an invalid value ("not_a_number", "not_a_bool", "not_json{{", etc.). Asserts load() succeeds and every non-string field
  equals its default.

Both tests serve as canaries: when adding a new field, adding it to the test YAML/env with a wrong type will immediately fail if the field lacks a graceful deserializer.

All 78 tests pass (cargo test -p datadog-agent-config --lib).

[Copilot]

Pull request overview

This PR improves the resilience of datadog-agent-config config loading by making deserialization failures non-fatal at the field level, so that a single mistyped YAML/env value no longer causes the entire source to fall back to Config::default().

Changes:

• Introduces a generic deserialize_with_default<T: Deserialize + Default> helper and applies it broadly via #[serde(deserialize_with)] on YAML/env config fields.
• Hardens several existing deserializers to catch and ignore serde/type errors instead of propagating them.
• Adds “canary” regression tests ensuring YAML/env loads succeed even when many fields have wrong types.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/datadog-agent-config/yaml.rs	Applies per-field graceful deserialization across YAML structs and adds a comprehensive wrong-type YAML regression test.
crates/datadog-agent-config/env.rs	Applies graceful deserialization for select env fields and adds a comprehensive wrong-type env regression test.
crates/datadog-agent-config/mod.rs	Adds deserialize_with_default and hardens multiple deserializers to ignore type/serde errors.
crates/datadog-agent-config/service_mapping.rs	Makes service mapping deserialization ignore type errors (returns empty map).
crates/datadog-agent-config/apm_replace_rule.rs	Makes APM replace rules deserialization ignore type errors (returns None).

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[apiarian-datadog]

some notes and questions