fix(agent): persist agent ID after auth to prevent crashloop duplicates

[wucm667]

Fixes #6527

Bug

When the Woodpecker agent is unable to query Docker, it crashes and restarts. Each restart creates a new agent entry on the server, leading to an infinite number of agent entries during crashloop.

Root Cause

The agent receives its assigned ID during authentication but only persists it after completing registration. If the agent crashes between auth and registration (e.g., Docker connection failure), the ID is lost. On restart, it authenticates as a new agent (ID 0) and gets a fresh server-side entry.

Fix

Persist the agent ID immediately after successful authentication in cmd/agent/core/agent.go. This ensures subsequent restarts reuse the same server-side entry via AgentUpdate instead of creating new ones.

Added AgentID() method to AuthClient to expose the agent ID received during auth.

Verification

• go build ./cmd/agent/... passes
• go test ./cmd/agent/... ./agent/... passes
• go test ./server/rpc/... -run TestRegisterAgent passes

[woodpecker-bot]

Account with automation patterns detected 🤖

@wucm667

We analyzed recent public events and found several patterns that are frequently associated with automated scripts or AI agents.

Human? 0% likely to be a human.

Signal	Points	Detail
Many recent forks	+51	8 repositories forked in a single day
High sustained fork rate	+55	9 repositories forked over 1 day (sustained high activity)

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_{🕵🏽 Analyzed 199 public events via AgentScan}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.52%. Comparing base (97eae44) to head (dfe48b2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6543      +/-   ##
==========================================
- Coverage   41.53%   41.52%   -0.01%     
==========================================
  Files         431      431              
  Lines       28725    28727       +2     
==========================================
  Hits        11930    11930              
- Misses      15721    15724       +3     
+ Partials     1074     1073       -1     

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