[config]: Support multiple named upstreams

[pseudomuto]

The proxy held a single Config.Upstream, which blocks the dynamic routing work (RFC #32): routing rules need to refer to upstreams by name, and a deployment serves more than one. This replaces it with Config.Upstreams, a named list, as the foundation for the routing block, engine, and connection registry that will follow.

Upstream targets may be templated (e.g. "{{ .RemoteNamespace }}.acme:7233") once rendering lands, so Upstream.Validate skips the literal IsHostPort check when the value contains a template action; a static hostPort is still validated. The inbound ListenConfig.Validate stays strict, since a listener address is never templated.

Config.PrimaryUpstream returns the first entry as an explicit, documented bridge rather than scatter an Upstreams[0] assumption across the fx modules; it will be removed when routing selects the upstream per request.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.97%. Comparing base (ad23f91) to head (2d64a9e).

Files with missing lines	Patch %	Lines
internal/proxy/fx.go	75.00%	1 Missing and 1 partial ⚠️
internal/router/fx.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
- Coverage   85.12%   84.97%   -0.16%     
==========================================
  Files          33       33              
  Lines        1042     1058      +16     
==========================================
+ Hits          887      899      +12     
- Misses        131      133       +2     
- Partials       24       26       +2     

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[Copilot]

Pull request overview

This PR evolves the proxy configuration from a single upstream to a list of named upstreams, enabling upcoming dynamic routing work (RFC #32) while providing a temporary “primary upstream” bridge for existing wiring.

Changes:

• Replace Config.Upstream with Config.Upstreams []Upstream and add Config.PrimaryUpstream() for transitional single-upstream consumers.
• Add Upstream.Name plus validation for required/unique names, and relax upstream hostPort validation when a value appears templated.
• Update fx modules/tests and dev config YAML to use the new upstreams schema.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/server/fx_test.go	Update server module tests to supply Upstreams instead of Upstream.
internal/router/fx.go	Switch router wiring to derive socket path from PrimaryUpstream().
internal/router/fx_test.go	Update router module tests to use Upstreams.
internal/proxy/fx.go	Resolve and validate the primary upstream before constructing the proxy; derive creds from selected upstream.
internal/proxy/fx_test.go	Update proxy module tests to use Upstreams.
internal/config/upstream.go	Add upstream Name and update validation (including templated hostPort handling).
internal/config/upstream_test.go	Adjust YAML fixtures and validation tests to the new upstreams schema.
internal/config/config.go	Introduce Upstreams, per-upstream validation, unique-name validation, and PrimaryUpstream().
internal/config/config_test.go	Extend validation coverage for missing/duplicate names and PrimaryUpstream().
dev/config.yaml	Update example dev configuration to use upstreams: list with named entry.